feat(admin): fingerprinting settings — on/off, length, match threshold, concurrency, sweep interval (M400 #3913)

Rule 25: the fingerprinting knobs move out of source into a DB-backed
singleton (migration 0061), edited from a card on the Duplicates page and
shared live with the scanner, the backfill and the duplicate sweep through
one service instance, so a save needs no restart.

The length is the knob that can silently break the library: prints taken
at two lengths never match. Each track_fingerprints row now records the
length it was taken at, and every reader filters on the current one — the
backfill treats another length as stale, the gauge counts it pending, the
sweep never streams it. Equivalent to a version bump, except that setting
the length back makes rows not yet redone current again. The card warns
before a length change re-fingerprints the library.

Off stops every decode: the scan takes only the stream hash (a demux, and
what recognises a moved file) and stores nothing, dropping a changed file's
stale row; the backfill idles. A save also makes a sweep due, since a new
threshold or length changes what the same prints group into, and the sweep
interval gains slack so an hourly interval on an hourly tick doesn't skip
every other tick.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SQ31KQpYbStyK5y58UmPLH

cmd/minstrel/main.go

@@ -122,7 +122,15 @@ func run() error {
 	}
 	defer pool.Close()
 
-	scanner := library.New(pool, logger, cfg.Library.ScanPaths)
+	// Fingerprinting settings (M400 #3913): one instance, shared by the scanner,
+	// the fingerprint backfill, the duplicate sweep and the admin API, so a save
+	// reaches all of them without a restart. A load failure is logged, not fatal:
+	// the service falls back to the shipped defaults.
+	fpSettings, fpErr := library.NewFingerprintSettingsService(ctx, pool)
+	if fpErr != nil {
+		logger.Warn("fingerprint settings: using defaults", "err", fpErr)
+	}
+	scanner := library.New(pool, logger, cfg.Library.ScanPaths, fpSettings)
 
 	contact := cfg.Library.ContactEmail
 	if contact == "" {
@@ -218,12 +226,12 @@ func run() error {
 	// will — everything imported before fingerprinting existed, and rows derived
 	// by an older method. A worker of its own rather than a scan stage; see
 	// internal/library/fingerprint_backfill.go for why.
-	go library.NewFingerprintBackfillWorker(pool, logger.With("component", "fingerprint_backfill")).Run(ctx)
+	go library.NewFingerprintBackfillWorker(pool, logger.With("component", "fingerprint_backfill"), fpSettings).Run(ctx)
 
 	// Duplicate sweep (M400 #3910): proposes groups of tracks holding one
 	// recording, from the fingerprints above. Sweeps only when fingerprints have
 	// changed since the last sweep.
-	go library.NewDuplicateSweepWorker(pool, logger.With("component", "duplicate_sweep")).Run(ctx)
+	go library.NewDuplicateSweepWorker(pool, logger.With("component", "duplicate_sweep"), fpSettings).Run(ctx)
 
 	// Start the tag-enrichment worker (#1490). Reconciles the compiled-in
 	// tag providers with tag_provider_settings, bumps the sources version if
@@ -368,6 +376,7 @@ func run() error {
 	srv.PlaylistScheduler = playlistScheduler
 	srv.RecSettings = recSettings
 	srv.TagSettings = tagSettings
+	srv.FingerprintSettings = fpSettings
 	srv.StreamSecret = cfg.StreamSecret
 	httpServer := &http.Server{
 		Addr:              cfg.Server.Address,