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/main.py

@@ -19,7 +19,10 @@ from .db import (
     update_revenue_record, delete_revenue_record, get_revenue_dashboard,
     get_market_trends as _get_market_trends,
     get_latest_trend_report, get_trend_reports as _get_trend_reports,
+    create_compile_job, get_compile_jobs, get_compile_job,
+    update_compile_job, delete_compile_job,
 )
+from .compiler import run_compile
 from .market import ingest_trends, get_suggestions
 from .local_provider import run_local_generation
 from .suno_provider import (
@@ -783,6 +786,66 @@ def delete_project(project_id: int):
     return {"ok": True}
 
 
+# ── Compile Jobs ──────────────────────────────────────────────────────────────
+
+class CompileRequest(BaseModel):
+    title: str = ""
+    track_ids: list[int]
+    crossfade_sec: float = 3.0
+
+
+@app.post("/api/music/compile")
+def create_compile(req: CompileRequest, background_tasks: BackgroundTasks):
+    if not req.track_ids:
+        raise HTTPException(status_code=400, detail="track_ids 필수")
+    if not (0.5 <= req.crossfade_sec <= 15):
+        raise HTTPException(status_code=400, detail="crossfade_sec: 0.5~15")
+    job_id = create_compile_job(req.title, req.track_ids, req.crossfade_sec)
+    background_tasks.add_task(run_compile, job_id)
+    return {"id": job_id, "status": "rendering"}
+
+
+@app.get("/api/music/compiles")
+def list_compiles():
+    return {"jobs": get_compile_jobs()}
+
+
+@app.get("/api/music/compile/{job_id}")
+def get_compile(job_id: int):
+    job = get_compile_job(job_id)
+    if not job:
+        raise HTTPException(status_code=404, detail="Not found")
+    return job
+
+
+@app.delete("/api/music/compile/{job_id}")
+def delete_compile(job_id: int):
+    job = get_compile_job(job_id)
+    if not job:
+        raise HTTPException(status_code=404, detail="Not found")
+    if job.get("output_path"):
+        out_dir = os.path.dirname(job["output_path"])
+        if os.path.exists(out_dir):
+            shutil.rmtree(out_dir, ignore_errors=True)
+    delete_compile_job(job_id)
+    return {"ok": True}
+
+
+@app.get("/api/music/compile/{job_id}/export")
+def export_compile(job_id: int):
+    job = get_compile_job(job_id)
+    if not job or job["status"] != "done":
+        raise HTTPException(status_code=404, detail="Not ready")
+    out_dir = os.path.dirname(job["output_path"])
+    rel = os.path.relpath(job["output_path"], os.getenv("VIDEO_DATA_DIR", "/app/data/videos"))
+    mp4_url = f"/media/videos/{rel}"
+    return {
+        "mp4_url":      mp4_url,
+        "duration_sec": job["duration_sec"],
+        "title":        job["title"],
+    }
+
+
 # ── 수익화 추적 API ───────────────────────────────────────────────────────────
 
 @app.get("/api/music/revenue/dashboard")