fix(设置): 修复站点设置保存失败的问题

问题：
1. Setting.value 字段设置了 NotEmpty() 约束，导致保存空字符串值时验证失败
2. 数据库 settings 表缺少 key 字段的唯一约束，导致 ON CONFLICT 语句执行失败

修复：
- 移除 ent/schema/setting.go 中 value 字段的 NotEmpty() 约束
- 新增迁移 015_fix_settings_unique_constraint.sql 添加缺失的唯一约束
- 添加3个回归测试确保空值保存功能正常

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/ent/runtime/runtime.go

@@ -415,10 +415,6 @@ func init() {
 			return nil
 		}
 	}()
-	// settingDescValue is the schema descriptor for value field.
-	settingDescValue := settingFields[1].Descriptor()
-	// setting.ValueValidator is a validator for the "value" field. It is called by the builders before save.
-	setting.ValueValidator = settingDescValue.Validators[0].(func(string) error)
 	// settingDescUpdatedAt is the schema descriptor for updated_at field.
 	settingDescUpdatedAt := settingFields[2].Descriptor()
 	// setting.DefaultUpdatedAt holds the default value on creation for the updated_at field.

backend/ent/schema/setting.go

@@ -36,7 +36,6 @@ func (Setting) Fields() []ent.Field {
 			NotEmpty().
 			Unique(),
 		field.String("value").
-			NotEmpty().
 			SchemaType(map[string]string{
 				dialect.Postgres: "text",
 			}),

backend/ent/setting/setting.go

@@ -44,8 +44,6 @@ func ValidColumn(column string) bool {
 var (
 	// KeyValidator is a validator for the "key" field. It is called by the builders before save.
 	KeyValidator func(string) error
-	// ValueValidator is a validator for the "value" field. It is called by the builders before save.
-	ValueValidator func(string) error
 	// DefaultUpdatedAt holds the default value on creation for the "updated_at" field.
 	DefaultUpdatedAt func() time.Time
 	// UpdateDefaultUpdatedAt holds the default value on update for the "updated_at" field.

backend/ent/setting_create.go

@@ -102,11 +102,6 @@ func (_c *SettingCreate) check() error {
 	if _, ok := _c.mutation.Value(); !ok {
 		return &ValidationError{Name: "value", err: errors.New(`ent: missing required field "Setting.value"`)}
 	}
-	if v, ok := _c.mutation.Value(); ok {
-		if err := setting.ValueValidator(v); err != nil {
-			return &ValidationError{Name: "value", err: fmt.Errorf(`ent: validator failed for field "Setting.value": %w`, err)}
-		}
-	}
 	if _, ok := _c.mutation.UpdatedAt(); !ok {
 		return &ValidationError{Name: "updated_at", err: errors.New(`ent: missing required field "Setting.updated_at"`)}
 	}

backend/ent/setting_update.go

@@ -110,11 +110,6 @@ func (_u *SettingUpdate) check() error {
 			return &ValidationError{Name: "key", err: fmt.Errorf(`ent: validator failed for field "Setting.key": %w`, err)}
 		}
 	}
-	if v, ok := _u.mutation.Value(); ok {
-		if err := setting.ValueValidator(v); err != nil {
-			return &ValidationError{Name: "value", err: fmt.Errorf(`ent: validator failed for field "Setting.value": %w`, err)}
-		}
-	}
 	return nil
 }
 
@@ -254,11 +249,6 @@ func (_u *SettingUpdateOne) check() error {
 			return &ValidationError{Name: "key", err: fmt.Errorf(`ent: validator failed for field "Setting.key": %w`, err)}
 		}
 	}
-	if v, ok := _u.mutation.Value(); ok {
-		if err := setting.ValueValidator(v); err != nil {
-			return &ValidationError{Name: "value", err: fmt.Errorf(`ent: validator failed for field "Setting.value": %w`, err)}
-		}
-	}
 	return nil
 }