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/api/server.go

@@ -264,31 +264,6 @@ func (s *Server) removeGarminClient(userID int64) {
 	}
 }
 
-// RunIncrementalSyncForAllUsers is called on a timer (see main.go) to sync
-// every provisioned user in turn, replacing the old single-global-Service
-// background loop.
-func (s *Server) RunIncrementalSyncForAllUsers(ctx context.Context) {
-	users, err := s.DB.ListUsers(ctx)
-	if err != nil {
-		log.Printf("api: list users for incremental sync: %v", err)
-		return
-	}
-	for _, u := range users {
-		svc, err := s.syncFor(ctx, u.ID)
-		if err != nil {
-			log.Printf("api: sync service for user %d: %v", u.ID, err)
-			continue
-		}
-		if err := svc.IncrementalSync(ctx); err != nil {
-			log.Printf("api: incremental sync for user %d: %v", u.ID, err)
-			continue
-		}
-		if err := svc.FillPendingDetails(ctx, 50); err != nil {
-			log.Printf("api: fill pending details for user %d: %v", u.ID, err)
-		}
-	}
-}
-
 var requestIDCounter atomic.Int64
 
 // requestLoggingMiddleware logs one JSON line per HTTP request (method,