store: scope GetProfile/UpdateProfile to a user_id

Part of per-user profile isolation: profile rows are no longer a global
singleton, so every read/write requires the caller's userID. This also
requires scoping ListActivities, ListWorkoutKinds, UpsertActivity,
CreateWorkoutKind, GetSyncState, UpdateSyncState, and ResetAllSyncedData
to userID, plus updating all related tests in the store package.

backend/internal/store/syncstate.go

@@ -13,25 +13,24 @@ type SyncState struct {
 	BackfillComplete   bool    // true once backfill has reached the configured horizon (or Garmin's own history start)
 }
 
-// GetSyncState returns the current backfill watermark (the singleton row,
-// created by migration 0002).
-func (db *DB) GetSyncState(ctx context.Context) (SyncState, error) {
+// GetSyncState returns the current backfill watermark for userID.
+func (db *DB) GetSyncState(ctx context.Context, userID int64) (SyncState, error) {
 	var s SyncState
-	err := db.QueryRowContext(ctx, `SELECT earliest_synced_date, backfill_complete FROM sync_state WHERE id = 1`).
+	err := db.QueryRowContext(ctx, `SELECT earliest_synced_date, backfill_complete FROM sync_state WHERE user_id = ?`, userID).
 		Scan(&s.EarliestSyncedDate, &s.BackfillComplete)
 	if err != nil {
-		return SyncState{}, fmt.Errorf("get sync state: %w", err)
+		return SyncState{}, fmt.Errorf("get sync state for user %d: %w", userID, err)
 	}
 	return s, nil
 }
 
-// UpdateSyncState records progress of a backfill run.
-func (db *DB) UpdateSyncState(ctx context.Context, earliestSyncedDate string, complete bool) error {
+// UpdateSyncState records progress of a backfill run for userID.
+func (db *DB) UpdateSyncState(ctx context.Context, userID int64, earliestSyncedDate string, complete bool) error {
 	_, err := db.ExecContext(ctx, `
-		UPDATE sync_state SET earliest_synced_date = ?, backfill_complete = ? WHERE id = 1`,
-		earliestSyncedDate, complete)
+		UPDATE sync_state SET earliest_synced_date = ?, backfill_complete = ? WHERE user_id = ?`,
+		earliestSyncedDate, complete, userID)
 	if err != nil {
-		return fmt.Errorf("update sync state: %w", err)
+		return fmt.Errorf("update sync state for user %d: %w", userID, err)
 	}
 	return nil
 }