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/server/services/PluginService.js

@@ -2,7 +2,7 @@ const mongoose = require('mongoose');
 const { encrypt, decrypt } = require('~/server/utils/');
 const { logger } = require('~/config');
 
-const PluginAuth = mongoose.models.PluginAuth;
+const PluginAuth = require('~/db/models').PluginAuth;
 
 /**
  * Asynchronously retrieves and decrypts the authentication value for a user's plugin, based on a specified authentication field.

api/server/services/UserService.js

@@ -4,7 +4,7 @@ const { ErrorTypes } = require('librechat-data-provider');
 const { encrypt, decrypt } = require('~/server/utils/crypto');
 const { updateUser } = require('~/models');
 
-const Key = mongoose.models.Key;
+const Key = require('~/db/models').Key;
 
 /**
  * Updates the plugins for a user based on the action specified (install/uninstall).

api/server/services/twoFactorService.js

@@ -2,7 +2,7 @@ const mongoose = require('mongoose');
 const { webcrypto } = require('node:crypto');
 const { hashBackupCode, decryptV3, decryptV2 } = require('~/server/utils/crypto');
 
-const User = mongoose.models.User;
+const User = require('~/db/models').User;
 
 // Base32 alphabet for TOTP secret encoding.
 const BASE32_ALPHABET = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ234567';