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/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"`)}
 	}