fix(13-02): auto-resolve phantom playlist tracks after library scan

- Add phantom_file_path column to playlist_tracks (migration 7)
- Store original file_path during RemoveLibrary phantom metadata population
- After each successful scan, UPDATE phantom tracks whose phantom_file_path
  now matches an audio_files row, re-linking them and clearing phantom metadata
- Update schema file, sqlc generated code, and database test for new column

backend/library/crud.go

@@ -244,10 +244,12 @@ func (l *Library) RemoveLibrary(id int64) (*RemovalSummary, error) {
 			phantom_album = sub.album,
 			phantom_duration_ms = sub.duration,
 			phantom_genre = sub.genre,
-			phantom_cover_art_path = sub.cover_art_path
+			phantom_cover_art_path = sub.cover_art_path,
+			phantom_file_path = sub.file_path
 		FROM (
 			SELECT
 				pt.id AS pt_id,
+				af.file_path AS file_path,
 				COALESCE(r.name, '') AS title,
 				COALESCE(ac.text, '') AS artist,
 				COALESCE(rg.name, '') AS album,

backend/library/library.go

@@ -652,7 +652,16 @@ func (l *Library) scanInternal(
 		metrics.OrphanCleanup = time.Since(orphanStart)
 	}
 
-	// --- Phase 6: post-scan variant generation ---
+	// --- Phase 6: resolve phantom playlist tracks ---
+	// Phantom tracks (audio_file_id IS NULL) that have a stored
+	// phantom_file_path matching a newly-scanned audio file are
+	// automatically re-linked.  This handles the case where a
+	// library directory is removed and later re-added.
+	if !cancelled {
+		l.resolvePhantomTracks()
+	}
+
+	// --- Phase 7: post-scan variant generation ---
 	if !cancelled {
 		variantStart := time.Now()
 
@@ -708,6 +717,53 @@ func (l *Library) scanInternal(
 	return metrics
 }
 
+// resolvePhantomTracks re-links phantom playlist_tracks entries
+// whose phantom_file_path now matches an audio_files row.  This
+// runs after every successful scan so that re-adding a previously
+// removed library automatically restores playlist references.
+func (l *Library) resolvePhantomTracks() {
+	// SAFETY: Hand-crafted UPDATE for phantom track resolution.
+	// Matches phantom playlist_tracks (audio_file_id IS NULL,
+	// phantom_file_path IS NOT NULL) against audio_files by
+	// file_path. Clears phantom metadata on resolved rows.
+	// No user input — all values come from the database.
+	result, err := l.db.ExecContext(`
+		UPDATE playlist_tracks SET
+			audio_file_id = (
+				SELECT af.id FROM audio_files af
+				WHERE af.file_path = playlist_tracks.phantom_file_path
+			),
+			phantom_title = NULL,
+			phantom_artist = NULL,
+			phantom_album = NULL,
+			phantom_duration_ms = NULL,
+			phantom_genre = NULL,
+			phantom_cover_art_path = NULL,
+			phantom_file_path = NULL
+		WHERE audio_file_id IS NULL
+			AND phantom_file_path IS NOT NULL
+			AND EXISTS (
+				SELECT 1 FROM audio_files af
+				WHERE af.file_path = playlist_tracks.phantom_file_path
+			)`)
+	if err != nil {
+		l.logger.Warn(
+			"could not resolve phantom playlist tracks",
+			"err", err,
+		)
+
+		return
+	}
+
+	resolved, _ := result.RowsAffected()
+	if resolved > 0 {
+		l.logger.Info(
+			"resolved phantom playlist tracks",
+			"count", resolved,
+		)
+	}
+}
+
 // progressInterval controls how often scan progress events are
 // emitted to the frontend.
 const progressInterval = 300 * time.Millisecond