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/Banner.js

@@ -1,7 +1,7 @@
 const mongoose = require('mongoose');
 const { logger } = require('@librechat/data-schemas');
 
-const Banner = mongoose.models.Banner;
+const Banner = require('~/db/models').Banner;
 
 /**
  * Retrieves the current active banner.