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/controller/auth_controller.go

@@ -57,6 +57,7 @@ func (ac *AuthController) RegisterPage(c *gin.Context) {
 		"ExtraCSS":            "/static/css/auth.css",
 		"Guidelines":          guidelines,
 		"RegistrationEnabled": ac.authService.IsRegistrationEnabled(),
+		"PasswordStrength":    ac.siteSettings.PasswordStrength(),
 		"ShowGuidelinesModal": ac.siteSettings.ShowGuidelinesModal(),
 		"ForceGuidelines":     ac.siteSettings.ForceGuidelines(),
 		"GuidelinesTimer":     ac.siteSettings.GuidelinesTimer(),