feat(client): App 内在线购买/续费授权（管理员支付宝下单+到账轮询）

- 新增套餐目录 license_plans.dart（标准/高级 × 月付/年付，与官网 checkout 一致）
- license 模型/仓库新增 PurchaseOrder/PurchaseStatusInfo 与下单、查单接口
- 系统设置授权面板：管理员显示「在线购买 / 续费」按钮，弹窗选套餐→
  外部浏览器打开支付宝收银台→3s 轮询到账→自动刷新授权并展示新到期日
- 用户手册（docs.md + user-manual.html）同步补充在线购买入口说明

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JJ1g8XV1YhhmHRzhwWEW7o

client/lib/models/license.dart

@@ -64,3 +64,57 @@ class LicenseInfo {
   bool get needsAttention =>
       phase == 'grace' || phase == 'readonly' || phase == 'locked';
 }
+
+/// POST /license/purchase 响应：pay 收银台跳转信息。
+class PurchaseOrder {
+  final String payUrl;
+  final String outTradeNo;
+  final String amount; // 实付金额（pay 侧权威价，如 "2999.00"）
+  final String subject;
+
+  const PurchaseOrder({
+    required this.payUrl,
+    required this.outTradeNo,
+    required this.amount,
+    required this.subject,
+  });
+
+  factory PurchaseOrder.fromJson(Map<String, dynamic> json) => PurchaseOrder(
+        payUrl: json['pay_url'] as String? ?? '',
+        outTradeNo: json['out_trade_no'] as String? ?? '',
+        amount: json['amount'] as String? ?? '',
+        subject: json['subject'] as String? ?? '',
+      );
+}
+
+/// GET /license/purchase/:otn 响应：购买单状态（到账轮询用）。
+class PurchaseStatusInfo {
+  final String outTradeNo;
+  final String status; // pending | paid | failed
+  final String amount;
+  final DateTime? paidAt;
+  final DateTime? expiresAt; // 续期后的门店授权到期时间（仅 paid 时有值）
+
+  const PurchaseStatusInfo({
+    required this.outTradeNo,
+    required this.status,
+    required this.amount,
+    this.paidAt,
+    this.expiresAt,
+  });
+
+  bool get isPaid => status == 'paid';
+
+  factory PurchaseStatusInfo.fromJson(Map<String, dynamic> json) =>
+      PurchaseStatusInfo(
+        outTradeNo: json['out_trade_no'] as String? ?? '',
+        status: json['status'] as String? ?? 'pending',
+        amount: json['amount'] as String? ?? '',
+        paidAt: json['paid_at'] != null
+            ? DateTime.tryParse(json['paid_at'] as String)
+            : null,
+        expiresAt: json['expires_at'] != null
+            ? DateTime.tryParse(json['expires_at'] as String)
+            : null,
+      );
+}