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/reset.go

@@ -5,23 +5,23 @@ import (
 	"fmt"
 )
 
-// ResetAllSyncedData deletes every synced activity (cascading to its laps,
+// ResetAllSyncedData deletes every synced activity for userID (cascading to its laps,
 // activity_samples, and kind_assignments via ON DELETE CASCADE) and rewinds
 // the backfill watermark to its initial state, so a subsequent Backfill
 // starts a genuinely fresh pull instead of thinking history is already
 // covered. Workout kinds (the user's taxonomy) are left untouched.
-func (db *DB) ResetAllSyncedData(ctx context.Context) error {
+func (db *DB) ResetAllSyncedData(ctx context.Context, userID int64) error {
 	tx, err := db.BeginTx(ctx, nil)
 	if err != nil {
 		return fmt.Errorf("begin reset tx: %w", err)
 	}
 	defer tx.Rollback()
 
-	if _, err := tx.ExecContext(ctx, `DELETE FROM activities`); err != nil {
-		return fmt.Errorf("delete activities: %w", err)
+	if _, err := tx.ExecContext(ctx, `DELETE FROM activities WHERE user_id = ?`, userID); err != nil {
+		return fmt.Errorf("delete activities for user %d: %w", userID, err)
 	}
-	if _, err := tx.ExecContext(ctx, `UPDATE sync_state SET earliest_synced_date = NULL, backfill_complete = 0 WHERE id = 1`); err != nil {
-		return fmt.Errorf("reset sync state: %w", err)
+	if _, err := tx.ExecContext(ctx, `UPDATE sync_state SET earliest_synced_date = NULL, backfill_complete = 0 WHERE user_id = ?`, userID); err != nil {
+		return fmt.Errorf("reset sync state for user %d: %w", userID, err)
 	}
 	return tx.Commit()
 }