feat: Payment 订单商品化——多态 payable 关联购买实体

- payments 表新增 payable_type/payable_id（nullableMorphs），订单可关联文章/套餐等可购买实体
- createOrder 接收 $payable 模型，取代 description 字符串解析
- membership 支付回调按 payable instanceof 分发（激活订阅 / 解锁单篇）
- Post/MembershipPlan 提供 payableLabel/payableUrl 约定方法；Payment 用约定方法生成展示标签与后台链接（皮肤等新实体可直接接入）
- 订单表格新增「类型」徽章（文章/会员套餐），商品列链接到对应后台编辑页；无 payable 的旧订单显示 —

tests/Feature/MembershipFlowTest.php

@@ -56,7 +56,8 @@ class MembershipFlowTest extends TestCase
         $payment = Payment::query()->where('user_id', $this->user->id)->first();
         $this->assertNotNull($payment);
         $this->assertSame('pending', $payment->status);
-        $this->assertSame('MembershipPlan#'.$this->plan->id, $payment->description);
+        $this->assertTrue($payment->payable->is($this->plan));
+        $this->assertNull($payment->description);
     }
 
     public function test_sandbox_payment_activates_subscription(): void
@@ -101,4 +102,49 @@ class MembershipFlowTest extends TestCase
             ->assertOk()
             ->assertSee('付费隐藏内容');
     }
+
+    public function test_unlock_post_creates_payment_with_payable_and_grants_access(): void
+    {
+        $post = Post::create([
+            'title' => '单篇付费',
+            'slug' => 'single-paid',
+            'content' => '付费内容正文',
+            'content_format' => 'markdown',
+            'status' => 'published',
+            'published_at' => now(),
+            'meta' => ['price' => 500],
+        ]);
+
+        $this->actingAs($this->user)
+            ->post('/posts/'.$post->id.'/unlock')
+            ->assertRedirect();
+
+        $payment = Payment::query()->where('user_id', $this->user->id)->first();
+        $this->assertNotNull($payment);
+        $this->assertTrue($payment->payable->is($post));
+        $this->assertSame(500, $payment->amount);
+        $this->assertStringContainsString('单篇付费', $payment->payable_label);
+        $this->assertNotNull($payment->payable_url);
+
+        // 沙箱支付后写入解锁用户
+        $this->actingAs($this->user)->post('/pay/sandbox/'.$payment->order_no.'/confirm');
+
+        $post->refresh();
+        $this->assertContains($this->user->id, $post->meta['unlocked_user_ids']);
+    }
+
+    public function test_legacy_order_without_payable_shows_placeholder(): void
+    {
+        $payment = Payment::create([
+            'order_no' => 'LEGACY0001',
+            'user_id' => $this->user->id,
+            'subject' => '历史订单',
+            'amount' => 100,
+            'status' => 'paid',
+            'paid_at' => now(),
+        ]);
+
+        $this->assertSame('—', $payment->payable_label);
+        $this->assertNull($payment->payable_url);
+    }
 }