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/db/models.js

@@ -0,0 +1,5 @@
+const mongoose = require('mongoose');
+const { createModels } = require('@librechat/data-schemas');
+const models = createModels(mongoose);
+
+module.exports = { ...models };