feat(library): fold a missing track into its on-disk replacement (M485 #5286 #5287)

A track marked missing whose replacement is already on disk under another
row — on the operator's library 355 of 451 missing tracks, nearly all Lidarr
mp3 -> flac upgrades — is folded into the replacement: likes, plays, playlist
entries and tags move across and the missing row goes. Move adoption could
not catch these: the replacements were re-encodes (no shared audio hash) with
no recording MBID at import.

Pairs (ListMissingTrackPairs): the same recording MBID within the album
group, or the same album row and title ignoring case. Each side must have
exactly one candidate; conflicting MBIDs refuse a pair. No duration or track
position gate: on the 142 pairs known to be one recording, 18% differed by
over 2s and the poorly tagged set is where numbering is broken (spike #5274).

Each pair folds in its own transaction after locking both rows and checking
the pair still holds. Runs automatically (operator, 2026-10-07) after a full
scan, after a watcher batch that added or updated tracks, and after an
AcoustID pass that matched any track. The first scan after deploy repairs the
existing rows.

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

internal/library/scanner.go

@@ -175,6 +175,14 @@ func (s *Scanner) Scan(ctx context.Context, progressCb func(Stats)) (Stats, erro
 	if err := ctx.Err(); err != nil {
 		return stats, err
 	}
+
+	// PHASE 4 — fold missing rows into replacements this scan brought in or
+	// that were already here (M485). After processing, so both this scan's
+	// marks and its arrivals are in. Not fatal: an unfolded pair is only a row
+	// left in the missing list, and the next scan tries again.
+	if _, err := PairMissingTracks(ctx, s.pool, s.logger); err != nil && ctx.Err() == nil {
+		s.logger.Warn("library scan: missing-pair pass failed", "err", err)
+	}
 	return stats, nil
 }
 
@@ -466,6 +474,11 @@ func (s *Scanner) ScanFiles(ctx context.Context, paths []string) ([]pgtype.UUID,
 			"skipped", stats.Skipped,
 			"errored", stats.Errored,
 		)
+		// A file the watcher just brought in may be the replacement for a row
+		// an earlier scan marked missing (M485).
+		if _, err := PairMissingTracks(ctx, s.pool, s.logger); err != nil && ctx.Err() == nil {
+			s.logger.Warn("library watch: missing-pair pass failed", "err", err)
+		}
 	}
 	return changed, nil
 }