feat(payment): 实现动态国家货币配置加载功能

- 移除硬编码国家货币映射，改为从API动态加载
- 添加 /api/payment/countries 接口获取支持的国家/货币列表
- 实现国家列表缓存机制（1小时有效期）
- 在支付页面添加国家选择下拉框动态渲染
- 添加网络请求失败时的备用数据方案
- 保留批量模式状态不被重置的注释说明

src/web/routes/payment.py

@@ -178,4 +178,52 @@ def mark_subscription(account_id: int, request: MarkSubscriptionRequest):
 
     return {"success": True, "subscription_type": request.subscription_type}
 
+# ============== 国家/货币配置 ==============
+
+_countries_cache: dict = {}  # {"data": [...], "expires_at": float}
+
+
+@router.get("/countries")
+def get_checkout_countries():
+    """从 ChatGPT checkout 接口获取支持的国家/货币列表（缓存 1 小时）"""
+    import time
+    import curl_cffi.requests as cffi_requests
+
+    now = time.time()
+    if _countries_cache.get("expires_at", 0) > now:
+        return {"success": True, "countries": _countries_cache["data"]}
+
+    with get_db() as db:
+        proxy = get_settings().get_proxy_url(db=db)
+
+    try:
+        resp = cffi_requests.get(
+            "https://chatgpt.com/backend-api/checkout_pricing_config/countries",
+            proxies={"http": proxy, "https": proxy} if proxy else None,
+            timeout=15,
+            impersonate="chrome110",
+        )
+        resp.raise_for_status()
+        data = resp.json()
+        countries = data if isinstance(data, list) else data.get("countries", [])
+        _countries_cache["data"] = countries
+        _countries_cache["expires_at"] = now + 3600
+        return {"success": True, "countries": countries}
+    except Exception as e:
+        logger.warning(f"获取国家列表失败: {e}")
+        fallback = [
+            {"country_code": "SG", "currency": "SGD", "country_name": "Singapore"},
+            {"country_code": "US", "currency": "USD", "country_name": "United States"},
+            {"country_code": "TR", "currency": "TRY", "country_name": "Turkey"},
+            {"country_code": "JP", "currency": "JPY", "country_name": "Japan"},
+            {"country_code": "HK", "currency": "HKD", "country_name": "Hong Kong"},
+            {"country_code": "GB", "currency": "GBP", "country_name": "United Kingdom"},
+            {"country_code": "AU", "currency": "AUD", "country_name": "Australia"},
+            {"country_code": "CA", "currency": "CAD", "country_name": "Canada"},
+            {"country_code": "IN", "currency": "INR", "country_name": "India"},
+            {"country_code": "BR", "currency": "BRL", "country_name": "Brazil"},
+            {"country_code": "MX", "currency": "MXN", "country_name": "Mexico"},
+        ]
+        return {"success": False, "countries": fallback, "error": str(e)}
+