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_processor.py

@@ -12,6 +12,9 @@ import uuid
 from dataclasses import dataclass
 from pathlib import Path
 
+# Constants
+UUID_LENGTH = 8  # UUID 截断长度
+
 # 尝试导入OCR库
 try:
     import pytesseract
@@ -340,7 +343,7 @@ class MultimodalProcessor:
         Returns:
             视频处理结果
         """
-        video_id = video_id or str(uuid.uuid4())[:8]
+        video_id = video_id or str(uuid.uuid4())[:UUID_LENGTH]
 
         try:
             # 保存视频文件
@@ -375,7 +378,7 @@ class MultimodalProcessor:
                 ocr_text, confidence = self.perform_ocr(frame_path)
 
                 frame = VideoFrame(
-                    id=str(uuid.uuid4())[:8],
+                    id=str(uuid.uuid4())[:UUID_LENGTH],
                     video_id=video_id,
                     frame_number=frame_number,
                     timestamp=timestamp,