feat: 添加音乐键位字段，更新相关数据结构和接口

backend/database/models/contents.gen.go

@@ -38,6 +38,7 @@ type Content struct {
 	CreatedAt           time.Time                `gorm:"column:created_at;type:timestamp with time zone;default:now()" json:"created_at"`
 	UpdatedAt           time.Time                `gorm:"column:updated_at;type:timestamp with time zone;default:now()" json:"updated_at"`
 	DeletedAt           gorm.DeletedAt           `gorm:"column:deleted_at;type:timestamp with time zone" json:"deleted_at"`
+	Key                 string                   `gorm:"column:key;type:character varying(32);comment:Musical key/tone" json:"key"` // Musical key/tone
 	Author              *User                    `gorm:"foreignKey:UserID;references:ID" json:"author,omitempty"`
 	ContentAssets       []*ContentAsset          `gorm:"foreignKey:ContentID;references:ID" json:"content_assets,omitempty"`
 	Comments            []*Comment               `gorm:"foreignKey:ContentID;references:ID" json:"comments,omitempty"`