Phase3: 三平台封面图生成

- cover_generator.py: Pillow生成知乎/微信/小红书封面图（渐变背景+标题）
- 知乎 1200×630 蓝调, 微信 900×500 绿调, 小红书 1080×1440 红调
- creator.py 流水线增加 cover_generator 作为最终回退
- db_helper.py 新增 save_cover_to_article
- main.py 挂载 /automation/images 静态目录
- articles.py preview 接口返回 images 字段
- topics.html 预览dialog展示封面图

platform/backend/app/api/articles.py

@@ -138,7 +138,7 @@ def preview_article(topic_id: str, platform: str = "zhihu", current_user: User =
     article = db.query(Article).filter(Article.id == article_id).first()
     if not article or not article.html_content:
         raise HTTPException(status_code=404, detail=f"Article not found for {topic_id} on {platform}")
-    return {"topic_id": topic_id, "platform": platform, "html": article.html_content}
+    return {"topic_id": topic_id, "platform": platform, "html": article.html_content, "images": article.images or {}}
 
 @router.put("/{topic_id}/content")
 def update_article_content(

platform/backend/app/main.py

@@ -95,6 +95,13 @@ app.include_router(platform_config.router)
 app.include_router(collector_mgmt.router)
 app.include_router(assistant.router)
 
+# 挂载自动生成的图片（必须先于前端根挂载）
+PROJECT_ROOT_DIR = Path(__file__).parent.parent.parent.parent
+AUTOMATION_IMAGES_DIR = PROJECT_ROOT_DIR / "automation" / "images"
+if AUTOMATION_IMAGES_DIR.exists():
+    app.mount("/automation/images", StaticFiles(directory=str(AUTOMATION_IMAGES_DIR)), name="images")
+    logger.info(f"Images mounted at /automation/images from {AUTOMATION_IMAGES_DIR}")
+
 # 挂载前端
 FRONTEND_DIR = Path(__file__).parent.parent.parent / "frontend"
 if FRONTEND_DIR.exists() and (FRONTEND_DIR / "index.html").exists():