feat: 密码强度多级校验 — low/medium/high/very_high

- site_settings 新增 password.strength 配置项
- 四级：low(len>=8) / medium(两类) / high(三类+无连续) / very_high(四类+无连续)
- 连续字符检查：重复/升序/降序/键盘水平/键盘垂直序列
- 注册和改密接口统一读取配置
- 前端 register.js 动态校验 + 管理后台安全设置区

internal/config/site_settings.go

@@ -166,6 +166,16 @@ func (s *SiteSettings) ForceGuidelines() bool {
 	return s.GetBool("registration.force_guidelines", true)
 }
 
+// PasswordStrength 密码强度级别
+func (s *SiteSettings) PasswordStrength() string {
+	level := s.Get("password.strength", "medium")
+	switch level {
+	case "low", "medium", "high", "very_high":
+		return level
+	}
+	return "medium"
+}
+
 // GuidelinesTimer 强制阅读倒计时秒数
 func (s *SiteSettings) GuidelinesTimer() int {
 	val := s.Get("registration.guidelines_timer", "60")