feat(client): pay 支付领域模型 + PaymentApi + 支付流控制器(轮询 activated)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013nMthbVEmQquxBRKb9Fj8u

client/lib/services/api_client.dart

@@ -91,12 +91,14 @@ class ApiClient {
   Never _throwFromResponse(http.Response resp) {
     String zh = '操作失败 (HTTP ${resp.statusCode})';
     String en = 'Request failed (HTTP ${resp.statusCode})';
+    String? code;
     try {
       final b = jsonDecode(resp.body) as Map<String, dynamic>;
       zh = b['message_zh'] as String? ?? zh;
       en = b['message_en'] as String? ?? en;
+      code = b['code'] as String?;
     } catch (_) {}
-    throw AuthApiException(statusCode: resp.statusCode, messageZh: zh, messageEn: en);
+    throw AuthApiException(statusCode: resp.statusCode, messageZh: zh, messageEn: en, code: code);
   }
 
   void dispose() => _client.close();

client/lib/services/auth_api.dart

@@ -14,12 +14,16 @@ class AuthApiException implements Exception {
     required this.statusCode,
     required this.messageZh,
     required this.messageEn,
+    this.code,
   });
 
   final int statusCode;
   final String messageZh;
   final String messageEn;
 
+  /// 服务端 apierr 的机器码(如 CURRENCY_MISMATCH),旧调用点可空。
+  final String? code;
+
   @override
   String toString() => 'AuthApiException($statusCode): $messageZh';
 }

client/lib/services/payment_api.dart

@@ -0,0 +1,38 @@
+// payment_api.dart — /v1/pay 代理端点封装(JWT 经 ApiClient 自动注入)。
+import '../models/payment.dart';
+import 'api_client.dart';
+
+class PaymentApi {
+  PaymentApi(this._c);
+  final ApiClient _c;
+
+  Future<List<PayCatalogItem>> catalog() async {
+    final body = await _c.getJson('/v1/pay/catalog');
+    final items = (body['items'] as List<dynamic>?) ?? const [];
+    return [for (final it in items) PayCatalogItem.fromJson(it as Map<String, dynamic>)];
+  }
+
+  Future<PayOrder> createOrder({
+    required String sku,
+    required String method,
+    Map<String, String>? metadata,
+  }) async =>
+      PayOrder.fromJson(await _c.postJson('/v1/pay/orders', {
+        'sku': sku,
+        'method': method,
+        if (metadata != null && metadata.isNotEmpty) 'metadata': metadata,
+      }));
+
+  Future<PayOrderStatus> orderStatus(String orderNo) async =>
+      PayOrderStatus.fromJson(await _c.getJson('/v1/pay/orders/$orderNo'));
+
+  Future<PayOrder> retry(String orderNo, {required String method, Map<String, String>? metadata}) async =>
+      PayOrder.fromJson(await _c.postJson('/v1/pay/orders/$orderNo/retry', {
+        'method': method,
+        if (metadata != null && metadata.isNotEmpty) 'metadata': metadata,
+      }));
+
+  Future<void> cancel(String orderNo) async {
+    await _c.postJson('/v1/pay/orders/$orderNo/cancel');
+  }
+}