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/Conversation.js

@@ -2,7 +2,7 @@ const mongoose = require('mongoose');
 const { logger } = require('@librechat/data-schemas');
 const { getMessages, deleteMessages } = require('./Message');
 
-const Conversation = mongoose.models.Conversation;
+const Conversation = require('~/db/models').Conversation;
 
 /**
  * Searches for a conversation by conversationId and returns a lean document with only conversationId and user.