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.