refactor(db): streamline model imports and remove unused model exports

- Removed the export of models from the database connection module to simplify the structure.
- Updated various files to import models directly from the new centralized models module.
- Ensured consistency across the codebase by replacing mongoose model references with the new import paths.

api/models/File.js

@@ -2,7 +2,7 @@ const mongoose = require('mongoose');
 const { logger } = require('@librechat/data-schemas');
 const { EToolResources } = require('librechat-data-provider');
 
-const File = mongoose.models.File;
+const File = require('~/db/models').File;
 
 /**
  * Finds a file by its file_id with additional query options.