feat: LicenseGuard 中间件 + Flutter license model/repo (21D+21E)

21D — 后端:
- Claims 新增 LicenseExpiresAt (*int64 unix 秒)，写入 JWT 避免每次查库
- middleware/license_guard.go: CalcLicensePhase / LicenseGuard / GetLicensePhase
  - grace(0-7d): 允许通行
  - readonly(7-15d): 拦截非 GET 写操作 → 403
  - locked(15d+): 全部拦截 → 403
- auth.go: issueTokens 在 JWT 中嵌入 license expires_at；Login 检查 locked 拒绝登录
- router: license/* 路由豁免 LicenseGuard（锁定时仍可激活/查状态）

21E — Flutter 前端:
- models/license.dart: 新 LicenseInfo，含 phase/maxDevices，去掉旧 activatedAt
- core/device/device_id.dart: 持久化 UUID-v4 作为设备 ID（SharedPreferences）
- repositories/license_repository.dart: getInfo/activate/deactivate，激活时携带设备信息
- providers/license_provider.dart: 改接 LicenseRepository
- settings_screen.dart: activatedAt 改为显示 maxDevices

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/lib/providers/license_provider.dart

@@ -1,53 +1,14 @@
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import '../core/api/api_client.dart';
 import '../core/auth/auth_state.dart';
+import '../models/license.dart';
+import '../repositories/license_repository.dart';
 
-class LicenseInfo {
-  final String type;       // trial / monthly / annual / lifetime
-  final bool isActive;
-  final DateTime? expiresAt;
-  final DateTime? activatedAt;
+export '../models/license.dart';
 
-  const LicenseInfo({
-    required this.type,
-    required this.isActive,
-    this.expiresAt,
-    this.activatedAt,
-  });
-
-  factory LicenseInfo.fromJson(Map<String, dynamic> json) {
-    return LicenseInfo(
-      type: json['type'] as String? ?? 'trial',
-      isActive: json['is_active'] as bool? ?? false,
-      expiresAt: json['expires_at'] != null
-          ? DateTime.tryParse(json['expires_at'] as String)
-          : null,
-      activatedAt: json['activated_at'] != null
-          ? DateTime.tryParse(json['activated_at'] as String)
-          : null,
-    );
-  }
-
-  String get typeLabel {
-    switch (type) {
-      case 'monthly': return '月度授权';
-      case 'annual':  return '年度授权';
-      case 'lifetime': return '永久授权';
-      default:        return '试用版';
-    }
-  }
-
-  /// 是否已过期
-  bool get isExpired =>
-      expiresAt != null && DateTime.now().isAfter(expiresAt!);
-
-  /// 距到期剩余天数（null = 永久）
-  int? get daysRemaining {
-    if (expiresAt == null) return null;
-    final diff = expiresAt!.difference(DateTime.now()).inDays;
-    return diff < 0 ? 0 : diff;
-  }
-}
+final licenseRepositoryProvider = Provider<LicenseRepository>(
+  (ref) => LicenseRepository(ref.read(apiClientProvider)),
+);
 
 final licenseProvider =
     AsyncNotifierProvider<LicenseNotifier, LicenseInfo?>(LicenseNotifier.new);
@@ -61,11 +22,7 @@ class LicenseNotifier extends AsyncNotifier<LicenseInfo?> {
 
   Future<LicenseInfo?> _fetch() async {
     try {
-      final client = ref.read(apiClientProvider);
-      final resp = await client.get('/license/info');
-      final data = resp.data['data'];
-      if (data == null) return null;
-      return LicenseInfo.fromJson(data as Map<String, dynamic>);
+      return await ref.read(licenseRepositoryProvider).getInfo();
     } catch (_) {
       return null;
     }