music-lab: 가사 저장/수정/삭제 CRUD API 추가

- saved_lyrics 테이블 (id, title, text, prompt, created_at, updated_at)
- GET /api/music/lyrics/library — 저장된 가사 목록 조회
- POST /api/music/lyrics/library — 가사 저장
- PUT /api/music/lyrics/library/:id — 가사 수정
- DELETE /api/music/lyrics/library/:id — 가사 삭제

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

music-lab/app/db.py

@@ -56,6 +56,18 @@ def init_db() -> None:
         """)
         conn.execute("CREATE INDEX IF NOT EXISTS idx_library_created ON music_library(created_at DESC)")
 
+        conn.execute("""
+            CREATE TABLE IF NOT EXISTS saved_lyrics (
+                id         INTEGER PRIMARY KEY AUTOINCREMENT,
+                title      TEXT NOT NULL DEFAULT '',
+                text       TEXT NOT NULL DEFAULT '',
+                prompt     TEXT NOT NULL DEFAULT '',
+                created_at TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ','now')),
+                updated_at TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ','now'))
+            )
+        """)
+        conn.execute("CREATE INDEX IF NOT EXISTS idx_lyrics_created ON saved_lyrics(created_at DESC)")
+
         # 기존 테이블 마이그레이션 (컬럼 없으면 추가)
         for col, default in [
             ("provider", "'local'"), ("lyrics", "''"),
@@ -219,3 +231,58 @@ def get_track_file_path(track_id: int) -> Optional[str]:
     with _conn() as conn:
         row = conn.execute("SELECT file_path FROM music_library WHERE id = ?", (track_id,)).fetchone()
     return row["file_path"] if row else None
+
+
+# ── saved_lyrics CRUD ────────────────────────────────────────────────────────
+
+def _lyrics_row_to_dict(r) -> Dict[str, Any]:
+    return {
+        "id":         r["id"],
+        "title":      r["title"],
+        "text":       r["text"],
+        "prompt":     r["prompt"],
+        "created_at": r["created_at"],
+        "updated_at": r["updated_at"],
+    }
+
+
+def get_all_lyrics() -> List[Dict[str, Any]]:
+    with _conn() as conn:
+        rows = conn.execute("SELECT * FROM saved_lyrics ORDER BY created_at DESC").fetchall()
+    return [_lyrics_row_to_dict(r) for r in rows]
+
+
+def add_lyrics(data: Dict[str, Any]) -> Dict[str, Any]:
+    with _conn() as conn:
+        conn.execute(
+            "INSERT INTO saved_lyrics (title, text, prompt) VALUES (?, ?, ?)",
+            (data.get("title", ""), data.get("text", ""), data.get("prompt", "")),
+        )
+        row = conn.execute("SELECT * FROM saved_lyrics WHERE rowid = last_insert_rowid()").fetchone()
+    return _lyrics_row_to_dict(row)
+
+
+def update_lyrics(lyrics_id: int, data: Dict[str, Any]) -> Optional[Dict[str, Any]]:
+    with _conn() as conn:
+        fields = []
+        values = []
+        for k in ("title", "text", "prompt"):
+            if k in data:
+                fields.append(f"{k} = ?")
+                values.append(data[k])
+        if not fields:
+            return None
+        fields.append("updated_at = strftime('%Y-%m-%dT%H:%M:%fZ','now')")
+        values.append(lyrics_id)
+        conn.execute(f"UPDATE saved_lyrics SET {', '.join(fields)} WHERE id = ?", values)
+        row = conn.execute("SELECT * FROM saved_lyrics WHERE id = ?", (lyrics_id,)).fetchone()
+    return _lyrics_row_to_dict(row) if row else None
+
+
+def delete_lyrics(lyrics_id: int) -> bool:
+    with _conn() as conn:
+        row = conn.execute("SELECT id FROM saved_lyrics WHERE id = ?", (lyrics_id,)).fetchone()
+        if not row:
+            return False
+        conn.execute("DELETE FROM saved_lyrics WHERE id = ?", (lyrics_id,))
+    return True