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/Assistant.js

@@ -1,5 +1,5 @@
 const mongoose = require('mongoose');
-const Assistant = mongoose.models.Assistant;
+const Assistant = require('~/db/models').Assistant;
 
 /**
  * Update an assistant with new data without overwriting existing properties,