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/balanceMethods.js

@@ -5,7 +5,7 @@ const { createAutoRefillTransaction } = require('./Transaction');
 const { logViolation } = require('~/cache');
 const { getMultiplier } = require('./tx');
 
-const Balance = mongoose.models.Balance;
+const Balance = require('~/db/models').Balance;
 
 function isInvalidDate(date) {
   return isNaN(date);