feat(insta-lab): 슬레이트 zip 패키지 다운로드 API (10 PNG + caption.txt)

GET /api/insta/slates/{slate_id}/package 엔드포인트 추가.
렌더된 card_assets PNG들 + suggested_caption + hashtags를
단일 zip으로 번들해 StreamingResponse 반환.
hashtags JSON 문자열/리스트 방어 파싱 포함.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

insta-lab/app/main.py

@@ -1,14 +1,16 @@
 """FastAPI entrypoint for insta-lab."""
 
 import asyncio
+import io
 import json
 import logging
 import os
+import zipfile
 from typing import Optional
 
 from fastapi import FastAPI, HTTPException, BackgroundTasks, Body, Query
 from fastapi.middleware.cors import CORSMiddleware
-from fastapi.responses import FileResponse
+from fastapi.responses import FileResponse, StreamingResponse
 from pydantic import BaseModel
 from _shared.access_log import install as install_access_log
 
@@ -247,6 +249,35 @@ def get_asset(slate_id: int, page: int):
     return FileResponse(match["file_path"], media_type="image/png")
 
 
+@app.get("/api/insta/slates/{slate_id}/package")
+def download_package(slate_id: int):
+    slate = db.get_card_slate(slate_id)
+    if not slate:
+        raise HTTPException(404, "slate not found")
+    assets = sorted(db.list_card_assets(slate_id), key=lambda a: a["page_index"])
+    if not assets:
+        raise HTTPException(409, "아직 렌더된 카드가 없습니다")
+    buf = io.BytesIO()
+    with zipfile.ZipFile(buf, "w", zipfile.ZIP_DEFLATED) as z:
+        for a in assets:
+            fp = a["file_path"]
+            if os.path.exists(fp):
+                z.write(fp, arcname=f"{a['page_index']:02d}.png")
+        caption = (slate.get("suggested_caption") or "").strip()
+        tags = slate.get("hashtags") or []
+        if isinstance(tags, str):
+            try:
+                tags = json.loads(tags)
+            except Exception:
+                tags = []
+        caption_full = caption + ("\n\n" + " ".join(tags) if tags else "")
+        z.writestr("caption.txt", caption_full)
+    buf.seek(0)
+    return StreamingResponse(buf, media_type="application/zip", headers={
+        "Content-Disposition": f'attachment; filename="insta_slate_{slate_id}.zip"'
+    })
+
+
 @app.delete("/api/insta/slates/{slate_id}")
 def delete_slate(slate_id: int):
     if not db.get_card_slate(slate_id):