- Delete dropIndexIfExists helper from `model/main.go`
- Remove all calls to dropIndexIfExists in `migrateDB` and `migrateDBFast`
- Drop related comments and MySQL-only DROP INDEX code paths
- Keep GORM AutoMigrate as the sole migration path for `Model` and `Vendor`
Why:
- Simplifies migrations and avoids destructive index drops at startup
- Prevents noisy MySQL 1091 errors and vendor-specific branches
- Aligns with composite unique indexes (uk_model_name_delete_at, uk_vendor_name_delete_at)
Impact:
- No expected runtime behavior change; schema remains managed by GORM
- Legacy single-column unique indexes (if any) will no longer be auto-dropped
- Safe across MySQL/PostgreSQL/SQLite; MySQL Chinese charset checks remain intact
Verification:
- Lint passed for `model/main.go`
- Confirmed no remaining `DROP INDEX` or `dropIndexIfExists` references
