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/convoStructure.spec.js

@@ -2,7 +2,7 @@ const mongoose = require('mongoose');
 const { MongoMemoryServer } = require('mongodb-memory-server');
 const { getMessages, bulkSaveMessages } = require('./Message');
 
-const Message = mongoose.models.Message;
+const Message = require('~/db/models').Message;
 
 // Original version of buildTree function
 function buildTree({ messages, fileMap }) {