feat: AI 周报摘要提示词自定义设置

- system_config 新增 ai_summary_prompt 配置键
- generate_summary 从数据库加载自定义提示词，为空则使用默认 SUMMARY_SYSTEM_PROMPT
- 系统设置新增「AI 周报提示词」卡片，支持编辑和清空恢复默认

Co-Authored-By: Claude <noreply@anthropic.com>

backend/app/api/system_config.py

@@ -66,7 +66,7 @@ async def update_config(
 ):
     """Update a single config key. Only directors can change settings."""
     value = body.get("value", "")
-    valid_keys = {"notification_time", "holidays"}
+    valid_keys = {"notification_time", "holidays", "ai_summary_prompt"}
 
     if key not in valid_keys:
         raise HTTPException(status_code=400, detail=f"不支持的配置项: {key}")