feat: 敏感操作增加IP维度限流

- RateLimiter 新增 AllowSensitive 方法（1分钟5次，超限封禁5分钟）
- 新增 SensitiveRateLimit Gin 中间件
- 改密（PUT /api/settings/password）增加限流保护
- 注销（POST /api/settings/delete-account）增加限流保护
- 签到（POST /api/level/checkin）增加限流保护

internal/router/deps_core.go

@@ -20,6 +20,7 @@ import (
 type dependencies struct {
 	authCtrl        *controller.AuthController
 	authMdw         *middleware.AuthMiddleware
+	rateLimiter     *middleware.RateLimiter
 	maintenanceMdw  *middleware.MaintenanceMiddleware
 	settingsCtrl    *controller.SettingsController
 	messageCtrl     *controller.MessageController