feat: add index on user.refer_code for faster invite code lookup

- Add idx_refer_code index to improve query performance
- Prevents full table scan when validating invite codes during registration

initialize/migrate/database/02130_add_refer_code_index.up.sql

@@ -0,0 +1,2 @@
+-- Add index on refer_code column for faster lookup
+ALTER TABLE `user` ADD INDEX `idx_refer_code` (`refer_code`);