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/dbtest/reset.go

@@ -130,4 +130,15 @@ func ResetDB(t *testing.T, pool *pgxpool.Pool) {
 	); err != nil {
 		t.Fatalf("dbtest.ResetDB reset tag-sources version: %v", err)
 	}
+	// Fingerprinting settings (M400 #3913), a singleton like the counters above.
+	// Every column goes back to its migration default rather than to literals
+	// written here, so a test can pin the Go defaults to the migration's.
+	if _, err := pool.Exec(ctx, `
+		UPDATE fingerprint_settings
+		   SET enabled = DEFAULT, chromaprint_length_sec = DEFAULT,
+		       acoustic_max_bit_error_rate = DEFAULT, backfill_concurrency = DEFAULT,
+		       sweep_interval_hours = DEFAULT, updated_at = DEFAULT`,
+	); err != nil {
+		t.Fatalf("dbtest.ResetDB reset fingerprint settings: %v", err)
+	}
 }