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/server/services/UserService.js

@@ -4,7 +4,7 @@ const { ErrorTypes } = require('librechat-data-provider');
 const { encrypt, decrypt } = require('~/server/utils/crypto');
 const { updateUser } = require('~/models');
 
-const Key = mongoose.models.Key;
+const Key = require('~/db/models').Key;
 
 /**
  * Updates the plugins for a user based on the action specified (install/uninstall).