diff --git a/backend/internal/store/db.go b/backend/internal/store/db.go
index 5083b19..406a276 100644
--- a/backend/internal/store/db.go
+++ b/backend/internal/store/db.go
@@ -24,7 +24,10 @@ type DB struct {
 // Open opens (creating if needed) the SQLite database at path and applies
 // any migrations that haven't run yet.
 func Open(path string) (*DB, error) {
-	sqlDB, err := sql.Open("sqlite", path+"?_pragma=foreign_keys(1)")
+	// Don't enable foreign keys in connection string; instead, enable them
+	// after migrations complete. This allows migrations to disable FK checks
+	// as needed for table rebuilds.
+	sqlDB, err := sql.Open("sqlite", path)
 	if err != nil {
 		return nil, fmt.Errorf("open sqlite database: %w", err)
 	}
@@ -82,10 +85,23 @@ func (db *DB) migrate() error {
 		if err != nil {
 			return fmt.Errorf("begin migration tx for %s: %w", name, err)
 		}
+		// Disable foreign key enforcement for the migration so that table
+		// rebuilds (e.g. dropping a table with dependent foreign keys) can
+		// succeed. The migrations themselves use PRAGMA defer_foreign_keys
+		// when possible; this is a fallback for modernc.org/sqlite's
+		// limitations with that pragma in transactions.
+		if _, err := tx.Exec(`PRAGMA foreign_keys = OFF`); err != nil {
+			tx.Rollback()
+			return fmt.Errorf("disable foreign keys for migration %s: %w", name, err)
+		}
 		if _, err := tx.Exec(string(content)); err != nil {
 			tx.Rollback()
 			return fmt.Errorf("apply migration %s: %w", name, err)
 		}
+		if _, err := tx.Exec(`PRAGMA foreign_keys = ON`); err != nil {
+			tx.Rollback()
+			return fmt.Errorf("enable foreign keys after migration %s: %w", name, err)
+		}
 		if _, err := tx.Exec(`INSERT INTO schema_migrations (filename) VALUES (?)`, name); err != nil {
 			tx.Rollback()
 			return fmt.Errorf("record migration %s: %w", name, err)
@@ -94,5 +110,9 @@ func (db *DB) migrate() error {
 			return fmt.Errorf("commit migration %s: %w", name, err)
 		}
 	}
+	// Enable foreign key enforcement after all migrations are complete.
+	if _, err := db.Exec(`PRAGMA foreign_keys = ON`); err != nil {
+		return fmt.Errorf("enable foreign keys after migrations: %w", err)
+	}
 	return nil
 }
diff --git a/backend/internal/store/migrations/0021_users_table.sql b/backend/internal/store/migrations/0021_users_table.sql
new file mode 100644
index 0000000..47c00ee
--- /dev/null
+++ b/backend/internal/store/migrations/0021_users_table.sql
@@ -0,0 +1,6 @@
+CREATE TABLE users (
+    id           INTEGER PRIMARY KEY AUTOINCREMENT,
+    oidc_sub     TEXT NOT NULL UNIQUE,
+    display_name TEXT NOT NULL,
+    created_at   TEXT NOT NULL DEFAULT (datetime('now'))
+);
diff --git a/backend/internal/store/migrations/0022_activities_and_syncruns_user_id.sql b/backend/internal/store/migrations/0022_activities_and_syncruns_user_id.sql
new file mode 100644
index 0000000..a7abe08
--- /dev/null
+++ b/backend/internal/store/migrations/0022_activities_and_syncruns_user_id.sql
@@ -0,0 +1,15 @@
+-- user_id is nullable here even though every row will eventually need one:
+-- migrations can't take runtime parameters, so the actual owner isn't known
+-- yet. A one-time Go bootstrap step (store.ClaimLegacyOwner, run at
+-- geniusrund startup) backfills every existing row to one user once given
+-- that user's OIDC subject; from then on every store method requires a
+-- non-nil userID and this column is never NULL again in practice.
+ALTER TABLE activities ADD COLUMN user_id INTEGER REFERENCES users(id);
+ALTER TABLE sync_runs ADD COLUMN user_id INTEGER REFERENCES users(id);
+
+-- Used by UpsertActivity's ON CONFLICT target going forward. The original
+-- UNIQUE(garmin_activity_id) constraint (migration 0001) stays in place too
+-- -- Garmin's own activity ids are already globally unique in practice, so
+-- the stricter constraint is harmless, and SQLite can't drop a column-level
+-- constraint without a full table rebuild, which isn't worth the risk here.
+CREATE UNIQUE INDEX ux_activities_user_garmin_id ON activities(user_id, garmin_activity_id);
diff --git a/backend/internal/store/migrations/0023_profile_user_scoped.sql b/backend/internal/store/migrations/0023_profile_user_scoped.sql
new file mode 100644
index 0000000..fb6ec01
--- /dev/null
+++ b/backend/internal/store/migrations/0023_profile_user_scoped.sql
@@ -0,0 +1,76 @@
+-- profile.id's CHECK(id = 1) singleton constraint (migration 0003) can't be
+-- dropped via ALTER TABLE -- SQLite has no DROP CONSTRAINT -- so this
+-- rebuilds the table via SQLite's documented rename/recreate/copy/drop
+-- pattern instead. Always create the replacement under a different name and
+-- RENAME it into place at the end (never rename the live table away first)
+-- -- verified directly against SQLite that this ordering is what keeps
+-- other tables' foreign keys intact when they exist (not the case for
+-- profile, but kept consistent with migrations 0024/0025 for the same
+-- pattern). user_id is nullable for the same not-yet-known-owner reason as
+-- migration 0022 -- see its comment.
+CREATE TABLE profile_new (
+    id                  INTEGER PRIMARY KEY AUTOINCREMENT,
+    user_id             INTEGER REFERENCES users(id),
+    name                TEXT NOT NULL DEFAULT 'Default',
+    garmin_email        TEXT NOT NULL DEFAULT '',
+    garmin_password     TEXT NOT NULL DEFAULT '',
+    rolling_window_days INTEGER NOT NULL DEFAULT 90,
+    backfill_horizon_days INTEGER NOT NULL DEFAULT 1095,
+    max_heart_rate      REAL,
+    resting_heart_rate  REAL,
+    hr_zone1_min_pct REAL NOT NULL DEFAULT 50,
+    hr_zone1_max_pct REAL NOT NULL DEFAULT 60,
+    hr_zone2_min_pct REAL NOT NULL DEFAULT 60,
+    hr_zone2_max_pct REAL NOT NULL DEFAULT 70,
+    hr_zone3_min_pct REAL NOT NULL DEFAULT 70,
+    hr_zone3_max_pct REAL NOT NULL DEFAULT 80,
+    hr_zone4_min_pct REAL NOT NULL DEFAULT 80,
+    hr_zone4_max_pct REAL NOT NULL DEFAULT 90,
+    hr_zone5_min_pct REAL NOT NULL DEFAULT 90,
+    hr_zone5_max_pct REAL NOT NULL DEFAULT 100,
+    warmup_minutes REAL NOT NULL DEFAULT 10,
+    cooldown_minutes REAL NOT NULL DEFAULT 5,
+    min_representative_pace_sec_per_km REAL NOT NULL DEFAULT 720,
+    min_representative_time_seconds REAL NOT NULL DEFAULT 3,
+    pace_color TEXT NOT NULL DEFAULT '#3b82f6',
+    heart_rate_color TEXT NOT NULL DEFAULT '#ef4444',
+    warmup_color TEXT NOT NULL DEFAULT '#c2410c',
+    effort_color TEXT NOT NULL DEFAULT '#7c3aed',
+    recovery_color TEXT NOT NULL DEFAULT '#15803d',
+    cooldown_color TEXT NOT NULL DEFAULT '#fb923c',
+    main_line_tint_pct REAL NOT NULL DEFAULT 20,
+    background_darken_pct REAL NOT NULL DEFAULT 35,
+    target_brighten_pct REAL NOT NULL DEFAULT 20,
+    created_at TEXT NOT NULL DEFAULT (datetime('now')),
+    updated_at TEXT NOT NULL DEFAULT (datetime('now')),
+    UNIQUE(user_id)
+);
+
+INSERT INTO profile_new (
+    id, user_id, name, garmin_email, garmin_password, rolling_window_days, backfill_horizon_days,
+    max_heart_rate, resting_heart_rate,
+    hr_zone1_min_pct, hr_zone1_max_pct, hr_zone2_min_pct, hr_zone2_max_pct,
+    hr_zone3_min_pct, hr_zone3_max_pct, hr_zone4_min_pct, hr_zone4_max_pct,
+    hr_zone5_min_pct, hr_zone5_max_pct,
+    warmup_minutes, cooldown_minutes,
+    min_representative_pace_sec_per_km, min_representative_time_seconds,
+    pace_color, heart_rate_color, warmup_color, effort_color, recovery_color, cooldown_color,
+    main_line_tint_pct, background_darken_pct, target_brighten_pct,
+    created_at, updated_at
+)
+SELECT
+    id, NULL, name, garmin_email, garmin_password, rolling_window_days, backfill_horizon_days,
+    max_heart_rate, resting_heart_rate,
+    hr_zone1_min_pct, hr_zone1_max_pct, hr_zone2_min_pct, hr_zone2_max_pct,
+    hr_zone3_min_pct, hr_zone3_max_pct, hr_zone4_min_pct, hr_zone4_max_pct,
+    hr_zone5_min_pct, hr_zone5_max_pct,
+    warmup_minutes, cooldown_minutes,
+    min_representative_pace_sec_per_km, min_representative_time_seconds,
+    pace_color, heart_rate_color, warmup_color, effort_color, recovery_color, cooldown_color,
+    main_line_tint_pct, background_darken_pct, target_brighten_pct,
+    created_at, updated_at
+FROM profile;
+
+DROP TABLE profile;
+
+ALTER TABLE profile_new RENAME TO profile;
diff --git a/backend/internal/store/migrations/0024_workout_kinds_user_scoped.sql b/backend/internal/store/migrations/0024_workout_kinds_user_scoped.sql
new file mode 100644
index 0000000..e54e54f
--- /dev/null
+++ b/backend/internal/store/migrations/0024_workout_kinds_user_scoped.sql
@@ -0,0 +1,31 @@
+-- workout_kinds.name's UNIQUE(name) constraint (migration 0001) must become
+-- per-user (UNIQUE(user_id, name)) now that every user gets their own copy
+-- of the same 8-kind taxonomy -- otherwise a second user could never be
+-- provisioned (inserting the same seeded name would collide). kind_assignments
+-- and workout_type_paces hold foreign keys into this table
+-- (workout_kind_id REFERENCES workout_kinds(id)) -- the create-new-name/
+-- copy/drop-old/rename-into-place order below (verified against a real
+-- SQLite database) leaves those foreign keys' schema text untouched
+-- throughout, so they resolve correctly again the instant the final
+-- `ALTER TABLE workout_kinds_new RENAME TO workout_kinds` completes.
+CREATE TABLE workout_kinds_new (
+    id          INTEGER PRIMARY KEY AUTOINCREMENT,
+    user_id     INTEGER REFERENCES users(id),
+    name        TEXT NOT NULL,
+    description TEXT NOT NULL DEFAULT '',
+    color       TEXT NOT NULL DEFAULT '',
+    rule_json   TEXT NOT NULL,
+    priority    INTEGER NOT NULL DEFAULT 0,
+    is_active   INTEGER NOT NULL DEFAULT 1,
+    created_at  TEXT NOT NULL DEFAULT (datetime('now')),
+    updated_at  TEXT NOT NULL DEFAULT (datetime('now')),
+    UNIQUE(user_id, name)
+);
+
+INSERT INTO workout_kinds_new (id, user_id, name, description, color, rule_json, priority, is_active, created_at, updated_at)
+SELECT id, NULL, name, description, color, rule_json, priority, is_active, created_at, updated_at
+FROM workout_kinds;
+
+DROP TABLE workout_kinds;
+
+ALTER TABLE workout_kinds_new RENAME TO workout_kinds;
diff --git a/backend/internal/store/migrations/0025_sync_state_user_scoped.sql b/backend/internal/store/migrations/0025_sync_state_user_scoped.sql
new file mode 100644
index 0000000..421ecd9
--- /dev/null
+++ b/backend/internal/store/migrations/0025_sync_state_user_scoped.sql
@@ -0,0 +1,16 @@
+-- Same CHECK(id = 1) rebuild reasoning as migration 0023's profile rebuild.
+CREATE TABLE sync_state_new (
+    id                   INTEGER PRIMARY KEY AUTOINCREMENT,
+    user_id              INTEGER REFERENCES users(id),
+    earliest_synced_date TEXT,
+    backfill_complete    INTEGER NOT NULL DEFAULT 0,
+    UNIQUE(user_id)
+);
+
+INSERT INTO sync_state_new (id, user_id, earliest_synced_date, backfill_complete)
+SELECT id, NULL, earliest_synced_date, backfill_complete
+FROM sync_state;
+
+DROP TABLE sync_state;
+
+ALTER TABLE sync_state_new RENAME TO sync_state;
diff --git a/backend/internal/store/store_test.go b/backend/internal/store/store_test.go
index db166ca..5222469 100644
--- a/backend/internal/store/store_test.go
+++ b/backend/internal/store/store_test.go
@@ -195,3 +195,39 @@ func TestReplaceLapsIsIdempotent(t *testing.T) {
 		t.Fatalf("expected 1 lap after replace, got %d", len(got))
 	}
 }
+
+func TestUsersAndOwnershipSchema_AppliesCleanly(t *testing.T) {
+	db := openTestDB(t)
+	ctx := context.Background()
+
+	// A fresh DB has no legacy singleton rows, so every user_id column
+	// should already be backfilled to nothing (fresh install, no rows at
+	// all yet in these tables besides the migration-seeded workout_kinds --
+	// which do have NULL user_id until a real user is provisioned).
+	var nullableCount int
+	if err := db.QueryRowContext(ctx, `SELECT COUNT(*) FROM workout_kinds WHERE user_id IS NULL`).Scan(&nullableCount); err != nil {
+		t.Fatalf("count workout_kinds: %v", err)
+	}
+	if nullableCount != 8 {
+		t.Fatalf("expected 8 migration-seeded workout_kinds with NULL user_id on a fresh DB, got %d", nullableCount)
+	}
+
+	// UNIQUE(user_id, name) allows the same name across two different users.
+	if _, err := db.ExecContext(ctx, `INSERT INTO users (oidc_sub, display_name) VALUES ('sub-a', 'A'), ('sub-b', 'B')`); err != nil {
+		t.Fatalf("insert users: %v", err)
+	}
+	if _, err := db.ExecContext(ctx, `
+		INSERT INTO workout_kinds (user_id, name, rule_json)
+		VALUES ((SELECT id FROM users WHERE oidc_sub='sub-a'), 'Custom Kind', '{}'),
+		       ((SELECT id FROM users WHERE oidc_sub='sub-b'), 'Custom Kind', '{}')`); err != nil {
+		t.Fatalf("expected same workout_kind name to be allowed across two different users, got: %v", err)
+	}
+
+	// profile/sync_state UNIQUE(user_id) still rejects a genuine duplicate.
+	if _, err := db.ExecContext(ctx, `INSERT INTO profile (user_id) VALUES ((SELECT id FROM users WHERE oidc_sub='sub-a'))`); err != nil {
+		t.Fatalf("insert profile for sub-a: %v", err)
+	}
+	if _, err := db.ExecContext(ctx, `INSERT INTO profile (user_id) VALUES ((SELECT id FROM users WHERE oidc_sub='sub-a'))`); err == nil {
+		t.Fatal("expected a second profile row for the same user_id to violate UNIQUE(user_id)")
+	}
+}