feat(server): retire scan scheduler for watcher + safety-net walk

Wire the fsnotify watcher and a fixed 12h safety-net delta walk in main;
remove the configurable scan scheduler (scheduler.go, scan_schedule table via
migration 0033, GET/PATCH /api/admin/scan/schedule, and the server/api
plumbing). Manual scan + scan status are unchanged.

internal/api/api.go

@@ -28,7 +28,7 @@ import (
 // Mount attaches /api/* handlers to r. Public endpoints (login) are outside
 // RequireUser; everything else is gated by the middleware. The events writer
 // is shared with the Subsonic mount so /rest/scrobble feeds the same store.
-func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playevents.Writer, recCfg config.RecommendationConfig, lidarrCfg *lidarrconfig.Service, lidarrReqs *lidarrrequests.Service, lidarrQuar *lidarrquarantine.Service, tracksSvc *tracks.Service, playlistsSvc *playlists.Service, coverEnricher *coverart.Enricher, coverSettings *coverart.SettingsService, scanner *library.Scanner, scanCfg library.RunScanConfig, scheduler *library.Scheduler, dataDir string, sender mailer.Sender, bus *eventbus.Bus, playlistScheduler *playlists.Scheduler, streamSecret []byte) {
+func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playevents.Writer, recCfg config.RecommendationConfig, lidarrCfg *lidarrconfig.Service, lidarrReqs *lidarrrequests.Service, lidarrQuar *lidarrquarantine.Service, tracksSvc *tracks.Service, playlistsSvc *playlists.Service, coverEnricher *coverart.Enricher, coverSettings *coverart.SettingsService, scanner *library.Scanner, scanCfg library.RunScanConfig, dataDir string, sender mailer.Sender, bus *eventbus.Bus, playlistScheduler *playlists.Scheduler, streamSecret []byte) {
 	rng := rand.New(rand.NewSource(rand.Int63()))
 	h := &handlers{
 		pool: pool, logger: logger, events: events, recCfg: recCfg,
@@ -42,7 +42,6 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 		coverSettings:     coverSettings,
 		scanner:           scanner,
 		scanCfg:           scanCfg,
-		scheduler:         scheduler,
 		dataDir:           dataDir,
 		mailer:            sender,
 		eventbus:          bus,
@@ -167,8 +166,6 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 
 				admin.Get("/scan/status", h.handleGetScanStatus)
 				admin.Post("/scan/run", h.handleTriggerScan)
-				admin.Get("/scan/schedule", h.handleGetScanSchedule)
-				admin.Patch("/scan/schedule", h.handlePatchScanSchedule)
 
 				admin.Get("/library/coverage", h.handleGetLibraryCoverage)
 
@@ -222,7 +219,6 @@ type handlers struct {
 	coverSettings     *coverart.SettingsService
 	scanner           *library.Scanner
 	scanCfg           library.RunScanConfig
-	scheduler         *library.Scheduler
 	dataDir           string
 	mailer            mailer.Sender
 	eventbus          *eventbus.Bus