🐛 fix(db): rename composite unique indexes to avoid drop/recreate on restart

- Model: rename `uk_model_name` -> `uk_model_name_delete_at`
  (composite on `model_name` + `deleted_at`)
- Vendor: rename `uk_vendor_name` -> `uk_vendor_name_delete_at`
  (composite on `name` + `deleted_at`)
- Keep legacy cleanup in `model/main.go` to drop old index names
  (`uk_model_name`, `model_name`, `uk_vendor_name`, `name`) for compatibility.

Result: idempotent GORM migrations and no unnecessary index churn on MySQL restarts.

Files:
- `model/model_meta.go`
- `model/vendor_meta.go`

model/model_meta.go

@@ -21,7 +21,7 @@ type BoundChannel struct {
 
 type Model struct {
 	Id          int            `json:"id"`
-	ModelName   string         `json:"model_name" gorm:"size:128;not null;uniqueIndex:uk_model_name,priority:1"`
+	ModelName   string         `json:"model_name" gorm:"size:128;not null;uniqueIndex:uk_model_name_delete_at,priority:1"`
 	Description string         `json:"description,omitempty" gorm:"type:text"`
 	Icon        string         `json:"icon,omitempty" gorm:"type:varchar(128)"`
 	Tags        string         `json:"tags,omitempty" gorm:"type:varchar(255)"`
@@ -30,7 +30,7 @@ type Model struct {
 	Status      int            `json:"status" gorm:"default:1"`
 	CreatedTime int64          `json:"created_time" gorm:"bigint"`
 	UpdatedTime int64          `json:"updated_time" gorm:"bigint"`
-	DeletedAt   gorm.DeletedAt `json:"-" gorm:"index;uniqueIndex:uk_model_name,priority:2"`
+	DeletedAt   gorm.DeletedAt `json:"-" gorm:"index;uniqueIndex:uk_model_name_delete_at,priority:2"`
 
 	BoundChannels []BoundChannel `json:"bound_channels,omitempty" gorm:"-"`
 	EnableGroups  []string       `json:"enable_groups,omitempty" gorm:"-"`