feat(library): AcoustID lookup worker fills the MBIDs tags leave empty (M401 #3920 #3921)

Migration 0069 adds tracks.mbid_source (tag | acoustid), a lookup state per
track (matched | ambiguous | no_match | failed) and the acoustid_settings
row (off, no key, min score 0.85).

The file's tag outranks a lookup (D4). UpsertTrack keeps a looked-up id
through a re-read that finds no tag id and replaces it as soon as one
appears. SetTrackMbidFromAcoustID refuses to write over a tag id.

The worker fingerprints each untagged track with fpcalc's compressed
print, looks it up and writes an id only when D5 settles it: one
recording at or above the threshold, or one left after matching title and
length. Ambiguous and no-match results write nothing. A key AcoustID
refuses, or the service being unreachable, stops the pass and is reported
in the worker's status. It never counts as a verdict on a track.

A changed file drops its lookup in the scan. The re-lookup takes back an
id that no longer matches.

Admin API: GET /api/admin/library/acoustid (settings, status, coverage by
source), PUT …/acoustid-settings (write-only key), POST …/acoustid/run,
GET …/acoustid/unsettled.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/library/fingerprint.go

@@ -270,6 +270,18 @@ func computeChromaprint(ctx context.Context, path string, lengthSec int32) ([]in
 	return parseFpcalcRaw(out)
 }
 
+// computeLookupFingerprint returns the compressed fingerprint and duration an
+// AcoustID lookup needs. It fails as runFingerprintTool does, so a stall or a
+// missing fpcalc reads as inconclusive (isInconclusive) and a file fpcalc
+// rejects as a verdict.
+func computeLookupFingerprint(ctx context.Context, path string) (lookupFingerprint, error) {
+	out, err := runFingerprintTool(ctx, "fpcalc", fpcalcLookupArgs(path))
+	if err != nil {
+		return lookupFingerprint{}, err
+	}
+	return parseFpcalcCompressed(out)
+}
+
 // runFingerprintTool runs one tool under fingerprintTimeout.
 //
 // Any non-zero exit is an error, and that deliberately includes fpcalc's exit 3: