fix(auth): replace submit turnstile widget with VerifyTurnstileForRegister

Port upstream's VerifyTurnstileForRegister which skips the duplicate
Turnstile check when email verify flow is already completed, instead of
requiring a second Turnstile widget on the verify page.

backend/internal/handler/auth_handler.go

@@ -113,8 +113,8 @@ func (h *AuthHandler) Register(c *gin.Context) {
 		return
 	}
 
-	// Turnstile 验证 — 始终执行，防止机器人自动化注册
-	if err := h.authService.VerifyTurnstile(c.Request.Context(), req.TurnstileToken, ip.GetClientIP(c)); err != nil {
+	// Turnstile 验证（邮箱验证码注册场景避免重复校验一次性 token）
+	if err := h.authService.VerifyTurnstileForRegister(c.Request.Context(), req.TurnstileToken, ip.GetClientIP(c), req.VerifyCode); err != nil {
 		response.ErrorFrom(c, err)
 		return
 	}