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/db/queries/fingerprints.sql

@@ -4,16 +4,17 @@
 -- older method. Replaces the row wholesale — a fingerprint of the old bytes has
 -- no standing once the file has changed.
 INSERT INTO track_fingerprints (
-    track_id, audio_stream_sha256, chromaprint, fingerprint_version
+    track_id, audio_stream_sha256, chromaprint, fingerprint_version, chromaprint_length_sec
 ) VALUES (
     sqlc.arg(track_id), sqlc.narg(audio_stream_sha256), sqlc.narg(chromaprint),
-    sqlc.arg(fingerprint_version)
+    sqlc.arg(fingerprint_version), sqlc.arg(chromaprint_length_sec)
 )
 ON CONFLICT (track_id) DO UPDATE SET
-    audio_stream_sha256 = EXCLUDED.audio_stream_sha256,
-    chromaprint         = EXCLUDED.chromaprint,
-    fingerprint_version = EXCLUDED.fingerprint_version,
-    computed_at         = now();
+    audio_stream_sha256    = EXCLUDED.audio_stream_sha256,
+    chromaprint            = EXCLUDED.chromaprint,
+    fingerprint_version    = EXCLUDED.fingerprint_version,
+    chromaprint_length_sec = EXCLUDED.chromaprint_length_sec,
+    computed_at            = now();
 
 -- name: DeleteTrackFingerprint :exec
 -- A file changed but could not be fingerprinted, for a reason unrelated to the
@@ -32,27 +33,37 @@ SELECT t.id, t.file_path
   FROM tracks t
   LEFT JOIN track_fingerprints f ON f.track_id = t.id
  WHERE t.missing_since IS NULL
-   AND (f.track_id IS NULL OR f.fingerprint_version < sqlc.arg(current_version))
+   -- A row taken at another length is as stale as one from an older method:
+   -- chromaprints at two lengths cannot be compared (#3913).
+   AND (f.track_id IS NULL
+        OR f.fingerprint_version < sqlc.arg(current_version)
+        OR f.chromaprint_length_sec <> sqlc.arg(chromaprint_length_sec))
    AND t.id > sqlc.arg(after_id)
  ORDER BY t.id
  LIMIT sqlc.arg(batch_limit);
 
 -- name: GetFingerprintCoverage :one
 -- The admin gauge for the backfill. fingerprinted + rejected + pending = total.
--- rejected is a row at the current version with a NULL half: a tool ran and
--- refused the file, which is settled rather than waiting. Missing tracks are
--- excluded, or the gauge could never reach the end.
+-- "Current" means derived by the current method AT the current length: a row at
+-- another length is pending, because the backfill will re-derive it. rejected is
+-- a current row with a NULL half: a tool ran and refused the file, which is
+-- settled rather than waiting. Missing tracks are excluded, or the gauge could
+-- never reach the end.
 SELECT count(*)::bigint AS total,
        count(*) FILTER (
            WHERE f.fingerprint_version >= sqlc.arg(current_version)
+             AND f.chromaprint_length_sec = sqlc.arg(chromaprint_length_sec)
              AND f.audio_stream_sha256 IS NOT NULL AND f.chromaprint IS NOT NULL
        )::bigint AS fingerprinted,
        count(*) FILTER (
            WHERE f.fingerprint_version >= sqlc.arg(current_version)
+             AND f.chromaprint_length_sec = sqlc.arg(chromaprint_length_sec)
              AND (f.audio_stream_sha256 IS NULL OR f.chromaprint IS NULL)
        )::bigint AS rejected,
        count(*) FILTER (
-           WHERE f.track_id IS NULL OR f.fingerprint_version < sqlc.arg(current_version)
+           WHERE f.track_id IS NULL
+              OR f.fingerprint_version < sqlc.arg(current_version)
+              OR f.chromaprint_length_sec <> sqlc.arg(chromaprint_length_sec)
        )::bigint AS pending
   FROM tracks t
   LEFT JOIN track_fingerprints f ON f.track_id = t.id