feat(server/m7-recurring-scans): admin schedule HTTP handlers

Two new endpoints under existing RequireAdmin middleware:
- GET /api/admin/scan/schedule — returns config + computed
  next_scheduled_at (ISO 8601; null when mode='off')
- PATCH /api/admin/scan/schedule — validates per-mode required
  fields, normalizes mode='off' to NULL the per-mode fields,
  writes via UpdateScanSchedule, calls scheduler.Refresh(), returns
  the updated record.

server.New + api.Mount gain a *library.Scheduler parameter; handlers
struct + Server struct gain the scheduler field. Test stubs
(server_test.go, library_test.go's Mount call) updated to pass nil.

Tests gated on MINSTREL_TEST_DATABASE_URL: GET default is mode=off
with null next; PATCH daily produces a 03:00 next-fire; PATCH
weekly without weekly_day → 400; PATCH off normalizes lingering
per-mode fields to NULL; non-admin → 403.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/minstrel/main.go

@@ -166,10 +166,9 @@ func run() error {
 	scheduler := library.NewScheduler(pool, logger.With("component", "scheduler"),
 		scanner, coverEnricher, scanCfg)
 	scheduler.Start(ctx)
-	_ = scheduler // keep reachable for future wiring (Task 4)
 	srv := server.New(logger, pool, scanner, subsonic.Config{
 		AllowPlaintextPassword: cfg.Subsonic.AllowPlaintextPassword,
-	}, cfg.Events, cfg.Recommendation, cfg.Storage.DataDir, cfg.Branding, coverEnricher, cfg.Library.CoverArtBackfillCap, coverSettings, scanner, scanCfg)
+	}, cfg.Events, cfg.Recommendation, cfg.Storage.DataDir, cfg.Branding, coverEnricher, cfg.Library.CoverArtBackfillCap, coverSettings, scanner, scanCfg, scheduler)
 	playlists.StartSystemPlaylistCron(ctx, pool, logger.With("component", "system_playlist_cron"))
 	httpServer := &http.Server{
 		Addr:              cfg.Server.Address,