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/Preset.js

@@ -1,7 +1,7 @@
 const mongoose = require('mongoose');
 const { logger } = require('@librechat/data-schemas');
 
-const Preset = mongoose.models.Preset;
+const Preset = require('~/db/models').Preset;
 
 const getPreset = async (user, presetId) => {
   try {