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/cmd/geniusrund/main.go

@@ -62,8 +62,6 @@ func main() {
 	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
 	defer stop()
 
-	go runIncrementalSyncLoop(ctx, server, cfg.IncrementalSyncEvery)
-
 	httpServer := &http.Server{Addr: cfg.Addr, Handler: server.Router()}
 	go func() {
 		log.Printf("geniusrund listening on %s", cfg.Addr)
@@ -80,19 +78,3 @@ func main() {
 		log.Printf("http server shutdown: %v", err)
 	}
 }
-
-// runIncrementalSyncLoop periodically syncs new activities for every
-// provisioned user in the background so the frontend doesn't need to
-// trigger every sync manually.
-func runIncrementalSyncLoop(ctx context.Context, server *api.Server, every time.Duration) {
-	ticker := time.NewTicker(every)
-	defer ticker.Stop()
-	for {
-		select {
-		case <-ctx.Done():
-			return
-		case <-ticker.C:
-			server.RunIncrementalSyncForAllUsers(ctx)
-		}
-	}
-}