feat(payment): implement secure 1:1 Polar customer binding system

- Add polar_cust_id column to users table for direct customer mapping
  - Rewrite PolarProvider customer logic to use stored customer IDs
  - Eliminate email-based customer lookup to prevent cross-user contamination
  - Implement self-healing mechanism for invalid customer IDs
  - Maintain external_id binding for Polar's reference system
  - Add comprehensive logging for customer lookup operations

app/Models/User.php

@@ -30,6 +30,7 @@ class User extends Authenticatable implements FilamentUser, MustVerifyEmail
         'email',
         'password',
         'level',
+        'polar_cust_id',
     ];
 
     /**