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

internal/library/scanner.go

@@ -79,11 +79,14 @@ type Scanner struct {
 	// integration test can substitute a deterministic one: CI has no real audio
 	// to fingerprint, and what the test pins is WHEN the scan fingerprints, not
 	// what the tools print. Call it through fingerprintFile.
-	fingerprint func(ctx context.Context, path string) fingerprintResult
+	fingerprint func(ctx context.Context, path string, opts fingerprintOptions) fingerprintResult
+	// settings is the operator's fingerprinting policy (#3913), shared with the
+	// workers and the admin API. Nil fingerprints with the defaults.
+	settings *FingerprintSettingsService
 }
 
-func New(pool *pgxpool.Pool, logger *slog.Logger, paths []string) *Scanner {
-	return &Scanner{pool: pool, logger: logger, paths: paths, fingerprint: computeFingerprint}
+func New(pool *pgxpool.Pool, logger *slog.Logger, paths []string, settings *FingerprintSettingsService) *Scanner {
+	return &Scanner{pool: pool, logger: logger, paths: paths, fingerprint: computeFingerprint, settings: settings}
 }
 
 // Scan walks every configured root and upserts any audio file whose mtime is
@@ -313,10 +316,18 @@ func (s *Scanner) scanFile(
 	//
 	// Computed before move adoption so adoption can match on the audio hash
 	// (#3914); stored after the upsert, once the row id is known.
+	//
+	// With fingerprinting switched off (#3913) the scan decodes nothing, but it
+	// still takes the stream hash: a demux rather than a decode, and the only
+	// thing that recognises a moved file with no MBID.
 	var fp fingerprintResult
+	fpCfg := s.settings.Get()
 	fingerprinted := !unchanged
 	if fingerprinted {
-		fp = s.fingerprintFile(ctx, path)
+		fp = s.fingerprintFile(ctx, path, fingerprintOptions{
+			lengthSec:   fpCfg.ChromaprintLengthSec,
+			chromaprint: fpCfg.Enabled,
+		})
 	}
 
 	// A path we've never seen might not be a new track — it might be one that
@@ -384,7 +395,13 @@ func (s *Scanner) scanFile(
 		s.logger.Warn("library scan: LogChange track upsert failed", "track_id", track.ID, "err", err)
 	}
 	if fingerprinted {
-		storeFingerprint(ctx, q, s.logger, track.ID, path, fp)
+		if fpCfg.Enabled {
+			storeFingerprint(ctx, q, s.logger, track.ID, path, fp, fpCfg.ChromaprintLengthSec)
+		} else if err := q.DeleteTrackFingerprint(ctx, track.ID); err != nil {
+			// Off, nothing is stored — but a row describing the previous bytes
+			// must not outlive them, or the sweep would compare audio that is gone.
+			s.logger.Warn("fingerprint: clearing stale fingerprint failed", "path", path, "err", err)
+		}
 	}
 
 	if knownTrack {