🧹 refactor(db): remove legacy models/vendors index cleanup logic

- 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

model/main.go

@@ -64,22 +64,6 @@ var DB *gorm.DB
 
 var LOG_DB *gorm.DB
 
-// dropIndexIfExists drops a MySQL index only if it exists to avoid noisy 1091 errors
-func dropIndexIfExists(tableName string, indexName string) {
-	if !common.UsingMySQL {
-		return
-	}
-	var count int64
-	// Check index existence via information_schema
-	err := DB.Raw(
-		"SELECT COUNT(1) FROM information_schema.statistics WHERE table_schema = DATABASE() AND table_name = ? AND index_name = ?",
-		tableName, indexName,
-	).Scan(&count).Error
-	if err == nil && count > 0 {
-		_ = DB.Exec("ALTER TABLE " + tableName + " DROP INDEX " + indexName + ";").Error
-	}
-}
-
 func createRootAccountIfNeed() error {
 	var user User
 	//if user.Status != common.UserStatusEnabled {
@@ -263,16 +247,6 @@ func InitLogDB() (err error) {
 }
 
 func migrateDB() error {
-	// 修复旧版本留下的唯一索引，允许软删除后重新插入同名记录
-	// 删除单列唯一索引（列级 UNIQUE）及早期命名方式，防止与新复合唯一索引 (model_name, deleted_at) 冲突
-	dropIndexIfExists("models", "uk_model_name") // 新版复合索引名称（若已存在）
-	dropIndexIfExists("models", "model_name")    // 旧版列级唯一索引名称
-
-	dropIndexIfExists("vendors", "uk_vendor_name") // 新版复合索引名称（若已存在）
-	dropIndexIfExists("vendors", "name")           // 旧版列级唯一索引名称
-	// 清理旧索引名（兼容历史），避免与新的复合唯一索引冲突
-	// 说明：仅清理旧名 uk_model_name/model_name、uk_vendor_name/name；新索引名 uk_model_name_delete_at/uk_vendor_name_delete_at 不在清理范围
-	// 计划：该兼容逻辑将在后续几个版本中移除
 	err := DB.AutoMigrate(
 		&Channel{},
 		&Token{},
@@ -299,14 +273,6 @@ func migrateDB() error {
 }
 
 func migrateDBFast() error {
-	// 清理旧索引名（兼容历史），允许软删除后重新插入同名记录
-	// 说明：仅清理旧名 uk_model_name/model_name、uk_vendor_name/name；新索引名 uk_model_name_delete_at/uk_vendor_name_delete_at 不在清理范围
-	// 计划：该兼容逻辑将在后续几个版本中移除
-	dropIndexIfExists("models", "uk_model_name")
-	dropIndexIfExists("models", "model_name")
-
-	dropIndexIfExists("vendors", "uk_vendor_name")
-	dropIndexIfExists("vendors", "name")
 
 	var wg sync.WaitGroup