store: add users table and user_id scoping to migrations

Schema-only step toward per-user profile isolation: profile/workout_kinds/
sync_state are rebuilt to drop their singleton constraints, activities/
sync_runs gain a nullable user_id column. Store methods are scoped in
later tasks.

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;