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/Project.js

@@ -1,7 +1,7 @@
 const mongoose = require('mongoose');
 const { GLOBAL_PROJECT_NAME } = require('librechat-data-provider').Constants;
 
-const Project = mongoose.models.Project;
+const Project = require('~/db/models').Project;
 
 /**
  * Retrieve a project by ID and convert the found project document to a plain object.