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/multimodal_entity_linker.py

@@ -8,6 +8,9 @@ import uuid
 from dataclasses import dataclass
 from difflib import SequenceMatcher
 
+# Constants
+UUID_LENGTH = 8  # UUID 截断长度
+
 # 尝试导入embedding库
 try:
     NUMPY_AVAILABLE = True
@@ -247,7 +250,7 @@ class MultimodalEntityLinker:
 
                     if result and result.matched_entity_id:
                         link = EntityLink(
-                            id=str(uuid.uuid4())[:8],
+                            id=str(uuid.uuid4())[:UUID_LENGTH],
                             project_id=project_id,
                             source_entity_id=ent1.get("id"),
                             target_entity_id=result.matched_entity_id,
@@ -461,7 +464,7 @@ class MultimodalEntityLinker:
             多模态实体记录
         """
         return MultimodalEntity(
-            id=str(uuid.uuid4())[:8],
+            id=str(uuid.uuid4())[:UUID_LENGTH],
             entity_id=entity_id,
             project_id=project_id,
             name="",  # 将在后续填充