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/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.