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/config/config.go

@@ -35,8 +35,7 @@ type Config struct {
 	// (see api.Server.garminFor), so it can never be silently left empty.
 	GarminTokenStoreRoot string
 
-	MinConfidence        float64
-	IncrementalSyncEvery time.Duration
+	MinConfidence float64
 	// LogLevel controls internal/applog's JSON logger ("debug"|"info"|"warn"|"error").
 	LogLevel string
 
@@ -76,7 +75,6 @@ func Load() (Config, error) {
 		GarminPythonPath:     getEnvDefault("GARMIN_WRAPPER_PYTHON", "python3"),
 		GarminTokenStoreRoot: os.Getenv("GARMIN_TOKENSTORE"),
 		MinConfidence:        getEnvFloat("GENIUSRUN_MIN_CONFIDENCE", 0.6),
-		IncrementalSyncEvery: getEnvDuration("GENIUSRUN_INCREMENTAL_SYNC_EVERY", 6*time.Hour),
 		LogLevel:             getEnvDefault("GENIUSRUN_LOG_LEVEL", "info"),
 		BackendURL:           strings.TrimRight(os.Getenv("GENIUSRUN_BACKEND_URL"), "/"),
 		OIDCIssuerURL:        os.Getenv("GENIUSRUN_OIDC_ISSUER_URL"),