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.down.sql

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+-- Remove index on refer_code column
+ALTER TABLE `user` DROP INDEX `idx_refer_code`;

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

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