修复心跳刷入rollback误用 + API参数规范化

- heartbeat_flush: 移除循环内session.rollback()，防止一个服务器
  刷入失败导致整个批次session状态损坏
- assets.py: 为Query参数添加Query()类型+描述+校验约束，
  统一FastAPI参数风格 (server_id/session_id/limit/parent_id)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/background/heartbeat_flush.py

@@ -91,7 +91,9 @@ async def heartbeat_flush_loop():
                         flushed += 1
                     except Exception as e:
                         logger.warning(f"Failed to flush server {key}: {e}")
-                        await session.rollback()
+                        # Don't rollback here — just skip this server and continue.
+                        # A partial UPDATE on a single server won't corrupt others.
+                        # The final commit() will persist all successful updates.
 
                 await session.commit()
                 logger.info(f"Heartbeat flush: {flushed}/{len(keys)} servers updated to MySQL")