feat(06-01): create track_metadata VIEW schema and migration 4

- Add track_metadata_view.sql for sqlc VIEW awareness
- Add migration4TrackMetadataView for existing databases
- sqlc generate produces TrackMetadatum model from VIEW
- migration2 inline JOIN preserved (runs before VIEW exists)

backend/database/sql/schemas/track_metadata_view.sql

@@ -0,0 +1,36 @@
+CREATE VIEW IF NOT EXISTS track_metadata AS
+SELECT
+    af.id,
+    af.file_path,
+    af.length_milliseconds,
+    COALESCE(r.name, '') AS title,
+    COALESCE(ac.text, '') AS artist_name,
+    r.track_number,
+    r.disc_number,
+    COALESCE(rg.name, '') AS album,
+    CAST(COALESCE(
+        (SELECT GROUP_CONCAT(g.name, '||')
+         FROM recording_genres rg_sub
+         JOIN genres g ON rg_sub.genre_id = g.id
+         WHERE rg_sub.recording_id = r.id),
+        ''
+    ) AS TEXT) AS genre,
+    COALESCE(r.year, 0) AS year,
+    COALESCE(r.composer, '') AS composer,
+    COALESCE(ft.extension, '') AS file_type,
+    af.sample_rate,
+    af.bit_depth,
+    af.channels,
+    af.bitrate,
+    af.file_size
+FROM audio_files af
+LEFT JOIN recordings r ON af.recording_id = r.id
+LEFT JOIN artist_credit ac ON r.artist_credit_id = ac.id
+LEFT JOIN (
+    SELECT recording_id,
+        MIN(release_group_id) AS release_group_id
+    FROM release_group_recordings
+    GROUP BY recording_id
+) rgr ON r.id = rgr.recording_id
+LEFT JOIN release_groups rg ON rgr.release_group_id = rg.id
+LEFT JOIN file_types ft ON af.file_type_id = ft.id;

backend/database/sql/sqlcgen/models.go

@@ -136,3 +136,23 @@ type SearchIndex struct {
 	Artist   string
 	Album    string
 }
+
+type TrackMetadatum struct {
+	ID                 int64
+	FilePath           string
+	LengthMilliseconds int64
+	Title              string
+	ArtistName         string
+	TrackNumber        sql.NullInt64
+	DiscNumber         sql.NullInt64
+	Album              string
+	Genre              string
+	Year               int64
+	Composer           string
+	FileType           string
+	SampleRate         int64
+	BitDepth           int64
+	Channels           int64
+	Bitrate            int64
+	FileSize           int64
+}