feat(server/m7-recurring-scans): Scheduler goroutine + boot wiring

Adds ScheduleConfig + NextFire (per-mode logic for off/interval/
daily/weekly) and the Scheduler struct + loop goroutine. The loop
reads scan_schedule on boot and on Refresh signals, computes
next-fire, sleeps via time.Timer, and calls TryStartScan when the
timer fires. Skip-on-conflict logging happens inside tryFire when
TryStartScan reports started=false with a non-stale in-flight row.

Boot wiring: cmd/minstrel/main.go constructs the scheduler after
coverEnricher and calls Start with the server's long-lived ctx.
The scheduler is not yet plumbed into the HTTP handlers (the next
task does that — handlers gain a scheduler field, admin schedule
endpoints land).

Tests cover the NextFire truth table (off / interval / daily today
+ tomorrow / weekly today-before-time + today-after-time + upcoming
day) plus invalid configs and parseHHMM edge cases. Refresh is
verified non-blocking via the buffered-channel + default pattern.

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

cmd/minstrel/main.go

@@ -163,6 +163,10 @@ func run() error {
 		ArtistEnrichCap: cfg.Library.CoverArtBackfillCap,
 		DataDir:         cfg.Storage.DataDir,
 	}
+	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)