refactor: remove automatic background incremental sync

Garmin's rate-limiting means every unattended sync attempt risks a
ban; syncing should only ever happen when explicitly triggered via
"Sync now" (POST /api/sync/run), never on an unattended timer.

Removes the periodic background loop (main.go's runIncrementalSyncLoop,
api.Server.RunIncrementalSyncForAllUsers), its GENIUSRUN_INCREMENTAL_SYNC_EVERY
config, and store.DB.ListUsers (which existed solely to feed it). The
manual "Sync now" flow (Backfill/IncrementalSync/FillPendingDetails via
FullSync) is untouched.

backend/internal/store/users.go

@@ -32,26 +32,6 @@ func (db *DB) GetUserBySub(ctx context.Context, oidcSub string) (User, bool, err
 	return u, true, nil
 }
 
-// ListUsers returns every provisioned user, for the background incremental
-// sync loop to iterate.
-func (db *DB) ListUsers(ctx context.Context) ([]User, error) {
-	rows, err := db.QueryContext(ctx, `SELECT id, oidc_sub, display_name, created_at FROM users ORDER BY id`)
-	if err != nil {
-		return nil, fmt.Errorf("list users: %w", err)
-	}
-	defer rows.Close()
-
-	users := []User{}
-	for rows.Next() {
-		var u User
-		if err := rows.Scan(&u.ID, &u.OIDCSub, &u.DisplayName, &u.CreatedAt); err != nil {
-			return nil, fmt.Errorf("scan user row: %w", err)
-		}
-		users = append(users, u)
-	}
-	return users, rows.Err()
-}
-
 // neverMatchRule is the placeholder every fresh install's rule-engine kinds
 // start with -- every activity lands in needs_review until the user tunes
 // real rules.