store: activities UNIQUE constraint rebuild for per-user design

Migration 0026 rebuilds the activities table to enforce UNIQUE(user_id,
garmin_activity_id) as the composite primary constraint, removing the old
global UNIQUE(garmin_activity_id) constraint from migration 0001. This
allows the same Garmin activity ID to appear for different users without
conflicts, which is essential for the per-user profile design and enables
the TestUpsertActivity_SameGarminActivityIDAllowedAcrossDifferentUsers
test to pass.

backend/internal/store/migrations/0026_activities_unique_constraint.sql

@@ -0,0 +1,46 @@
+-- activities.garmin_activity_id's UNIQUE constraint (migration 0001) must become
+-- per-user (UNIQUE(user_id, garmin_activity_id)) now that the per-user-profile
+-- design allows multiple users to share the same Garmin activity ID. The old
+-- constraint alone was intentionally left in place by migration 0022 as a
+-- belts-and-suspenders safeguard (Garmin IDs are globally unique in practice),
+-- but it must be removed now to allow Task 6's cross-user test to pass.
+PRAGMA foreign_keys = OFF;
+
+CREATE TABLE activities_new (
+    id                         INTEGER PRIMARY KEY AUTOINCREMENT,
+    user_id                    INTEGER REFERENCES users(id),
+    garmin_activity_id         INTEGER NOT NULL,
+    event_type_key             TEXT NOT NULL,
+    workout_id                 INTEGER,
+    start_time_utc             TEXT NOT NULL,
+    duration_seconds           REAL NOT NULL,
+    distance_meters            REAL NOT NULL,
+    avg_hr                     REAL,
+    max_hr                     REAL,
+    avg_speed_mps              REAL,
+    elevation_gain_m           REAL,
+    aerobic_training_effect    REAL,
+    anaerobic_training_effect  REAL,
+    vo2max_value               REAL,
+    raw_json                   TEXT NOT NULL,
+    details_fetched_at         TEXT,
+    details_raw_json           TEXT,
+    splits_fetched_at          TEXT,
+    workout_raw_json           TEXT,
+    created_at                 TEXT NOT NULL DEFAULT (datetime('now')),
+    updated_at                 TEXT NOT NULL DEFAULT (datetime('now')),
+    UNIQUE(user_id, garmin_activity_id)
+);
+
+INSERT INTO activities_new (id, user_id, garmin_activity_id, event_type_key, workout_id, start_time_utc, duration_seconds, distance_meters, avg_hr, max_hr, avg_speed_mps, elevation_gain_m, aerobic_training_effect, anaerobic_training_effect, vo2max_value, raw_json, details_fetched_at, details_raw_json, splits_fetched_at, workout_raw_json, created_at, updated_at)
+SELECT id, user_id, garmin_activity_id, event_type_key, workout_id, start_time_utc, duration_seconds, distance_meters, avg_hr, max_hr, avg_speed_mps, elevation_gain_m, aerobic_training_effect, anaerobic_training_effect, vo2max_value, raw_json, details_fetched_at, details_raw_json, splits_fetched_at, workout_raw_json, created_at, updated_at
+FROM activities;
+
+DROP TABLE activities;
+
+ALTER TABLE activities_new RENAME TO activities;
+
+CREATE INDEX idx_activities_start_time ON activities(start_time_utc);
+CREATE INDEX idx_activities_user_garmin_id ON activities(user_id, garmin_activity_id);
+
+PRAGMA foreign_keys = ON;