refactor(api): /v1/me 收敛 expire_at → 只发 expires_at(无 web 用户中心)

之前后端同时发 expire_at + expires_at(分别给 app + web 用户中心)。现已无 web 端,
双发是纯冗余,统一只用 expires_at:
- account.go:meResponse 删 ExpireAt 字段 + GetMe handler 不再赋值
- client me.dart:fromJson 删 (expires_at ?? expire_at) 别名回退
- 两端契约快照同步:Go meResponse 冻结集去 expire_at(12→11 key)、删 Dart 别名测试
- 验证:go test httpapi 绿、flutter test contract(14)绿、analyze EXIT 0、e2e 全链路绿

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/lib/models/me.dart

@@ -43,7 +43,7 @@ class Me {
   bool get isFree => plan == 'free';
 
   factory Me.fromJson(Map<String, dynamic> m) {
-    final exp = (m['expires_at'] ?? m['expire_at']) as String?;
+    final exp = m['expires_at'] as String?;
     return Me(
       email: m['email'] as String? ?? '',
       plan: m['plan'] as String? ?? 'free',

client/test/contract/api_contract_test.dart

@@ -87,14 +87,6 @@ void main() {
       expect(me.weeklyGb, hasLength(7));
       expect(me.totpEnabled, isTrue);
     });
-
-    test('expires_at 接受别名 expire_at(后端历史兼容)', () {
-      final me = Me.fromJson({
-        ..._meSample()..remove('expires_at'),
-        'expire_at': '2027-01-01T00:00:00Z',
-      });
-      expect(me.expiresAt, DateTime.tryParse('2027-01-01T00:00:00Z'));
-    });
   });
 
   group('/v1/usage 契约快照(UsagePoint)', () {