feat(music-lab): 다중 트랙 컴파일 백엔드 (FFmpeg concat+crossfade → MP4)

- db.py: compile_jobs 테이블 추가 + CRUD 5종 (create/get/list/update/delete)
- compiler.py: acrossfade 필터 체인 + 그라디언트 배경 + MP4 렌더링 워커
- main.py: /api/music/compile POST·GET·DELETE + /api/music/compiles GET + /api/music/compile/{id}/export GET

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

music-lab/app/db.py

@@ -169,6 +169,21 @@ def init_db() -> None:
             )
         """)
 
+        # ── compile_jobs 테이블 ───────────────────────────────────────────
+        conn.execute("""
+            CREATE TABLE IF NOT EXISTS compile_jobs (
+                id            INTEGER PRIMARY KEY AUTOINCREMENT,
+                title         TEXT    NOT NULL DEFAULT '',
+                track_ids     TEXT    NOT NULL DEFAULT '[]',
+                crossfade_sec REAL    NOT NULL DEFAULT 3.0,
+                status        TEXT    NOT NULL DEFAULT 'pending',
+                output_path   TEXT,
+                duration_sec  REAL,
+                error         TEXT,
+                created_at    TEXT    NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ','now'))
+            )
+        """)
+
 
 # ── music_tasks CRUD ──────────────────────────────────────────────────────────
 
@@ -707,3 +722,72 @@ def get_trend_reports(limit: int = 10) -> list:
         }
         for r in rows
     ]
+
+
+# ── Compile Jobs ─────────────────────────────────────────
+
+def create_compile_job(title: str, track_ids: list, crossfade_sec: float) -> int:
+    with _conn() as conn:
+        cur = conn.execute(
+            "INSERT INTO compile_jobs (title, track_ids, crossfade_sec) VALUES (?,?,?)",
+            (title, json.dumps(track_ids), crossfade_sec),
+        )
+        return cur.lastrowid
+
+
+def get_compile_jobs() -> list:
+    with _conn() as conn:
+        rows = conn.execute(
+            "SELECT id, title, track_ids, crossfade_sec, status, duration_sec, created_at "
+            "FROM compile_jobs ORDER BY created_at DESC LIMIT 50"
+        ).fetchall()
+    return [
+        {
+            "id":            r["id"],
+            "title":         r["title"],
+            "track_ids":     json.loads(r["track_ids"]),
+            "crossfade_sec": r["crossfade_sec"],
+            "status":        r["status"],
+            "duration_sec":  r["duration_sec"],
+            "created_at":    r["created_at"],
+        }
+        for r in rows
+    ]
+
+
+def get_compile_job(job_id: int) -> Optional[Dict[str, Any]]:
+    with _conn() as conn:
+        r = conn.execute(
+            "SELECT * FROM compile_jobs WHERE id = ?", (job_id,)
+        ).fetchone()
+    if not r:
+        return None
+    return {
+        "id":            r["id"],
+        "title":         r["title"],
+        "track_ids":     json.loads(r["track_ids"]),
+        "crossfade_sec": r["crossfade_sec"],
+        "status":        r["status"],
+        "output_path":   r["output_path"],
+        "duration_sec":  r["duration_sec"],
+        "error":         r["error"],
+        "created_at":    r["created_at"],
+    }
+
+
+def update_compile_job(job_id: int, **kwargs) -> None:
+    allowed = {"status", "output_path", "duration_sec", "error"}
+    fields = {k: v for k, v in kwargs.items() if k in allowed}
+    if not fields:
+        return
+    set_clause = ", ".join(f"{k} = ?" for k in fields)
+    with _conn() as conn:
+        conn.execute(
+            f"UPDATE compile_jobs SET {set_clause} WHERE id = ?",
+            (*fields.values(), job_id),
+        )
+
+
+def delete_compile_job(job_id: int) -> None:
+    with _conn() as conn:
+        conn.execute("DELETE FROM compile_jobs WHERE id = ?", (job_id,))