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

scripts/db_helper.py

@@ -259,6 +259,27 @@ def save_article(topic_id: str, platform: str, html_content: str, db: Optional[S
         if close_db:
             db.close()
 
+def save_cover_to_article(topic_id: str, platform: str, cover_path: str, db: Optional[Session] = None):
+    """保存封面图路径到 article.images 字段"""
+    close_db = False
+    if db is None:
+        db = SessionLocal()
+        close_db = True
+    try:
+        from app.models import Article
+        article_id = f"{platform}_{topic_id}"
+        article = db.query(Article).filter(Article.id == article_id).first()
+        if article:
+            images = article.images or {}
+            images["cover"] = cover_path
+            article.images = images
+            db.commit()
+    except Exception:
+        db.rollback()
+    finally:
+        if close_db:
+            db.close()
+
 def get_article(topic_id: str, platform: str, db: Optional[Session] = None) -> Optional[Dict]:
     """从 articles 表获取文章 HTML"""
     close_db = False