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/llm_client.py

@@ -15,11 +15,13 @@ import httpx
 KIMI_API_KEY = os.getenv("KIMI_API_KEY", "")
 KIMI_BASE_URL = os.getenv("KIMI_BASE_URL", "https://api.kimi.com/coding")
 
+
 @dataclass
 class ChatMessage:
     role: str
     content: str
 
+
 @dataclass
 class EntityExtractionResult:
     name: str
@@ -27,6 +29,7 @@ class EntityExtractionResult:
     definition: str
     confidence: float
 
+
 @dataclass
 class RelationExtractionResult:
     source: str
@@ -34,6 +37,7 @@ class RelationExtractionResult:
     type: str
     confidence: float
 
+
 class LLMClient:
     """Kimi API 客户端"""
 
@@ -163,7 +167,7 @@ class LLMClient:
                 for r in data.get("relations", [])
             ]
             return entities, relations
-        except Exception as e:
+        except (RuntimeError, ValueError, TypeError) as e:
             print(f"Parse extraction result failed: {e}")
             return [], []
 
@@ -254,9 +258,11 @@ class LLMClient:
         messages = [ChatMessage(role="user", content=prompt)]
         return await self.chat(messages, temperature=0.3)
 
+
 # Singleton instance
 _llm_client = None
 
+
 def get_llm_client() -> LLMClient:
     global _llm_client
     if _llm_client is None: