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/models/license.dart

@@ -0,0 +1,56 @@
+class LicenseInfo {
+  final int id;
+  final String type; // trial | monthly | annual | lifetime
+  final bool isActive;
+  final int maxDevices;
+  final DateTime? expiresAt;
+  final String phase; // normal | grace | readonly | locked
+
+  const LicenseInfo({
+    required this.id,
+    required this.type,
+    required this.isActive,
+    required this.maxDevices,
+    required this.phase,
+    this.expiresAt,
+  });
+
+  factory LicenseInfo.fromJson(Map<String, dynamic> json) {
+    return LicenseInfo(
+      id: (json['id'] as num?)?.toInt() ?? 0,
+      type: json['type'] as String? ?? 'trial',
+      isActive: json['is_active'] as bool? ?? false,
+      maxDevices: (json['max_devices'] as num?)?.toInt() ?? 3,
+      expiresAt: json['expires_at'] != null
+          ? DateTime.tryParse(json['expires_at'] as String)
+          : null,
+      phase: json['phase'] as String? ?? 'normal',
+    );
+  }
+
+  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';
+    }
+  }
+
+  bool get isExpired =>
+      expiresAt != null && DateTime.now().isAfter(expiresAt!);
+
+  int? get daysRemaining {
+    if (expiresAt == null) return null;
+    final diff = expiresAt!.difference(DateTime.now()).inDays;
+    return diff < 0 ? 0 : diff;
+  }
+
+  bool get isReadOnlyPhase => phase == 'readonly' || phase == 'locked';
+  bool get isLockedPhase => phase == 'locked';
+  bool get needsAttention => phase == 'grace' || phase == 'readonly' || phase == 'locked';
+}