feat(backend): 完善日志系统，支持按日期分目录存储

- 实现 logs/YYYY-MM-DD/ 日期文件夹结构
- 添加 success.log 和 failure.log 专用日志
- 使用 TimedRotatingFileHandler 实现按天切割
- 添加 log_success 和 log_failure 便捷函数
- 集成 markitdown 进行文件转换
- 优化文件存储路径，按项目ID分类存储

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

backend/app/main.py

@@ -107,7 +107,7 @@ async def app_exception_handler(request: Request, exc: AppException):
         content=ApiResponse.fail(
             message=exc.message,
             error={"code": exc.code, "details": exc.details}
-        ).model_dump()
+        ).model_dump(mode='json')
     )
 
 
@@ -127,7 +127,7 @@ async def validation_exception_handler(request: Request, exc: RequestValidationE
         content=ApiResponse.fail(
             message="Validation error",
             error={"code": "VALIDATION_ERROR", "details": {"errors": errors}}
-        ).model_dump()
+        ).model_dump(mode='json')
     )
 
 
@@ -140,7 +140,7 @@ async def database_exception_handler(request: Request, exc: SQLAlchemyError):
         content=ApiResponse.fail(
             message="Database operation failed",
             error={"code": "DATABASE_ERROR"}
-        ).model_dump()
+        ).model_dump(mode='json')
     )
 
 
@@ -153,7 +153,7 @@ async def general_exception_handler(request: Request, exc: Exception):
         content=ApiResponse.fail(
             message="Internal server error",
             error={"code": "INTERNAL_ERROR"}
-        ).model_dump()
+        ).model_dump(mode='json')
     )