fix: auto-fix code issues (cron)

- 修复重复导入/字段
- 修复异常处理 (将裸 except Exception 改为具体异常类型)
- 修复PEP8格式问题
- 清理未使用导入
- 添加 UUID_LENGTH 常量替代魔法数字
- 添加 DEFAULT_RATE_LIMIT, MASTER_KEY_RATE_LIMIT, IP_RATE_LIMIT 常量
- 添加 MAX_TEXT_LENGTH, DEFAULT_TIMEOUT 常量

涉及文件:
- backend/main.py
- backend/db_manager.py
- backend/llm_client.py
- backend/neo4j_manager.py
- backend/tingwu_client.py
- backend/tenant_manager.py
- backend/growth_manager.py
- backend/workflow_manager.py
- backend/image_processor.py
- backend/multimodal_entity_linker.py
- backend/multimodal_processor.py
- backend/plugin_manager.py
- backend/rate_limiter.py

backend/growth_manager.py

@@ -456,7 +456,7 @@ class GrowthManager:
                 await client.post(
                     "https://api.mixpanel.com/track", headers=headers, json=[payload], timeout=10.0
                 )
-        except Exception as e:
+        except (RuntimeError, ValueError, TypeError) as e:
             print(f"Failed to send to Mixpanel: {e}")
 
     async def _send_to_amplitude(self, event: AnalyticsEvent):
@@ -484,7 +484,7 @@ class GrowthManager:
                     json=payload,
                     timeout=10.0,
                 )
-        except Exception as e:
+        except (RuntimeError, ValueError, TypeError) as e:
             print(f"Failed to send to Amplitude: {e}")
 
     async def _update_user_profile(