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/compiler.py

@@ -0,0 +1,121 @@
+import os
+import subprocess
+
+from .db import get_compile_job, get_track_file_path, get_track_by_id, update_compile_job
+
+VIDEO_DATA_DIR = os.getenv("VIDEO_DATA_DIR", "/app/data/videos")
+
+
+def _build_concat_cmd(file_paths: list[str], crossfade_sec: float, output_path: str) -> list:
+    """FFmpeg command: N audio files → single audio with acrossfade."""
+    n = len(file_paths)
+    if n == 1:
+        # Single track: just copy
+        return ["ffmpeg", "-y", "-i", file_paths[0], "-c:a", "libmp3lame", "-q:a", "2", output_path]
+
+    cmd = []
+    for fp in file_paths:
+        cmd += ["-i", fp]
+
+    # Build acrossfade filter chain
+    filter_parts = []
+    prev = "0"
+    for i in range(1, n):
+        out_label = f"a{i:02d}"
+        filter_parts.append(f"[{prev}][{i}]acrossfade=d={crossfade_sec}:c1=tri:c2=tri[{out_label}]")
+        prev = out_label
+
+    filter_str = ";".join(filter_parts)
+
+    return (
+        ["ffmpeg", "-y"]
+        + cmd
+        + ["-filter_complex", filter_str, "-map", f"[{prev}]", "-c:a", "libmp3lame", "-q:a", "2", output_path]
+    )
+
+
+def _make_gradient_bg(width: int, height: int, output_path: str) -> None:
+    """Simple dark gradient background image via FFmpeg."""
+    subprocess.run(
+        [
+            "ffmpeg", "-y",
+            "-f", "lavfi",
+            "-i", f"color=c=0x111827:size={width}x{height}:rate=1",
+            "-frames:v", "1",
+            output_path,
+        ],
+        check=True, capture_output=True,
+    )
+
+
+def _get_audio_duration(path: str) -> float:
+    """Return duration in seconds via ffprobe."""
+    try:
+        result = subprocess.run(
+            ["ffprobe", "-v", "error", "-show_entries", "format=duration",
+             "-of", "default=noprint_wrappers=1:nokey=1", path],
+            capture_output=True, text=True, check=True,
+        )
+        return float(result.stdout.strip())
+    except Exception:
+        return 0.0
+
+
+def run_compile(job_id: int) -> None:
+    """Main compile worker — called as BackgroundTask."""
+    job = get_compile_job(job_id)
+    if not job:
+        return
+
+    update_compile_job(job_id, status="rendering")
+
+    try:
+        track_ids = job["track_ids"]
+        if not track_ids:
+            raise ValueError("트랙이 선택되지 않았습니다")
+
+        # Resolve file paths
+        file_paths = []
+        for tid in track_ids:
+            fp = get_track_file_path(tid)
+            if not fp or not os.path.exists(fp):
+                raise ValueError(f"트랙 파일 없음 (id={tid})")
+            file_paths.append(fp)
+
+        out_dir = os.path.join(VIDEO_DATA_DIR, f"compile_{job_id}")
+        os.makedirs(out_dir, exist_ok=True)
+
+        # Step 1: concat audio
+        audio_path = os.path.join(out_dir, "audio.mp3")
+        concat_cmd = _build_concat_cmd(file_paths, job["crossfade_sec"], audio_path)
+        subprocess.run(concat_cmd, check=True, capture_output=True)
+
+        duration = _get_audio_duration(audio_path)
+
+        # Step 2: background image
+        bg_path = os.path.join(out_dir, "bg.jpg")
+        _make_gradient_bg(1920, 1080, bg_path)
+
+        # Step 3: audio + bg → MP4
+        output_path = os.path.join(out_dir, "output.mp4")
+        subprocess.run(
+            [
+                "ffmpeg", "-y",
+                "-loop", "1", "-i", bg_path,
+                "-i", audio_path,
+                "-c:v", "libx264", "-tune", "stillimage", "-preset", "fast",
+                "-c:a", "aac", "-b:a", "192k",
+                "-pix_fmt", "yuv420p",
+                "-shortest",
+                output_path,
+            ],
+            check=True, capture_output=True,
+        )
+
+        update_compile_job(job_id, status="done", output_path=output_path, duration_sec=duration)
+
+    except subprocess.CalledProcessError as e:
+        err = (e.stderr or b"").decode()[-300:]
+        update_compile_job(job_id, status="failed", error=err)
+    except Exception as e:
+        update_compile_job(job_id, status="failed", error=str(e))