feat(music-lab): AI 커버 생성 + 그라데이션 폴백

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

music-lab/app/pipeline/cover.py

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+"""AI 커버 아트 생성 — DALL·E 3 / gpt-image-1 + 그라데이션 폴백."""
+import base64
+import logging
+import os
+from io import BytesIO
+
+import httpx
+from PIL import Image
+
+from . import storage
+from .gradient import make_gradient_with_title
+
+logger = logging.getLogger("music-lab.cover")
+
+DALLE_TIMEOUT_S = 90
+
+
+def _get_api_key() -> str:
+    return os.getenv("OPENAI_API_KEY", "")
+
+
+def _get_model() -> str:
+    return os.getenv("OPENAI_IMAGE_MODEL", "gpt-image-1")
+
+
+async def generate(*, pipeline_id: int, genre: str, prompt_template: str,
+                   mood: str = "", track_title: str = "", feedback: str = "") -> dict:
+    """커버 아트 생성. 성공 시 jpg 저장 + URL 반환. 실패 시 그라데이션 폴백.
+
+    반환: {"url": str, "used_fallback": bool, "error": str | None}
+    """
+    out_path = os.path.join(storage.pipeline_dir(pipeline_id), "cover.jpg")
+    used_fallback = False
+    error = None
+
+    api_key = _get_api_key()
+    model = _get_model()
+    if api_key:
+        try:
+            await _generate_with_dalle(prompt_template, mood, feedback, out_path,
+                                       api_key=api_key, model=model)
+        except (httpx.HTTPError, httpx.TimeoutException, KeyError, ValueError, OSError) as e:
+            logger.warning("DALL·E 실패 — 폴백: %s", e)
+            error = str(e)
+            used_fallback = True
+            make_gradient_with_title(genre, track_title, out_path)
+    else:
+        used_fallback = True
+        error = "OPENAI_API_KEY 미설정"
+        make_gradient_with_title(genre, track_title, out_path)
+
+    return {
+        "url": storage.media_url(pipeline_id, "cover.jpg"),
+        "used_fallback": used_fallback,
+        "error": error,
+    }
+
+
+async def _generate_with_dalle(prompt_template: str, mood: str,
+                                feedback: str, out_path: str,
+                                *, api_key: str, model: str) -> None:
+    prompt = prompt_template
+    if mood:
+        prompt = f"{prompt}, {mood} mood"
+    if feedback:
+        prompt = f"{prompt}. 추가 지시: {feedback}"
+    prompt = f"{prompt}, no text, high quality"
+
+    async with httpx.AsyncClient(timeout=DALLE_TIMEOUT_S) as client:
+        resp = await client.post(
+            "https://api.openai.com/v1/images/generations",
+            headers={"Authorization": f"Bearer {api_key}"},
+            json={"model": model, "prompt": prompt, "size": "1024x1024", "n": 1},
+        )
+        resp.raise_for_status()
+        data = resp.json()["data"][0]
+        if "url" in data:
+            img_resp = await client.get(data["url"])
+            img_resp.raise_for_status()
+            img_bytes = img_resp.content
+        elif "b64_json" in data:
+            img_bytes = base64.b64decode(data["b64_json"])
+        else:
+            raise ValueError("DALL·E response has neither url nor b64_json")
+        # PNG → JPG 변환
+        with Image.open(BytesIO(img_bytes)) as src:
+            img = src.convert("RGB")
+        img.save(out_path, "JPEG", quality=92)