PlatformOrder create: lock merchant/plan when subscription context present

resources/views/admin/platform_orders/form.blade.php

@@ -79,27 +79,45 @@
         <input type="hidden" name="back" value="{{ $backValSafe }}">
     @endif
 
+    @php
+        $hasLockedSubscription = (bool) (($siteSubscription ?? null) && ($siteSubscription->id ?? 0));
+        $merchantIdVal = (string) old('merchant_id', $defaults['merchant_id'] ?? '');
+        $planIdVal = (string) old('plan_id', $defaults['plan_id'] ?? '');
+    @endphp
+
     <label>
         <span>站点</span>
-        <select name="merchant_id" required>
+        @if($hasLockedSubscription)
+            <input type="hidden" name="merchant_id" value="{{ $merchantIdVal }}">
+        @endif
+        <select name="merchant_id" required @disabled($hasLockedSubscription)>
             <option value="">请选择站点</option>
             @foreach(($merchants ?? []) as $m)
-                <option value="{{ $m->id }}" @selected((string)old('merchant_id', $defaults['merchant_id'] ?? '') === (string)$m->id)>{{ $m->name }}</option>
+                <option value="{{ $m->id }}" @selected($merchantIdVal === (string)$m->id)>{{ $m->name }}</option>
             @endforeach
         </select>
+        @if($hasLockedSubscription)
+            <small class="muted">已从订阅上下文锁定站点（避免误选导致续费串单）。</small>
+        @endif
     </label>
 
     <label>
         <span>套餐</span>
-        <select name="plan_id" required>
+        @if($hasLockedSubscription)
+            <input type="hidden" name="plan_id" value="{{ $planIdVal }}">
+        @endif
+        <select name="plan_id" required @disabled($hasLockedSubscription)>
             <option value="">请选择套餐</option>
             @foreach(($plans ?? []) as $p)
-                <option value="{{ $p->id }}" @selected((string)old('plan_id', $defaults['plan_id'] ?? '') === (string)$p->id)>
+                <option value="{{ $p->id }}" @selected($planIdVal === (string)$p->id)>
                     {{ $p->name }}（{{ $billingCycleLabels[$p->billing_cycle] ?? $p->billing_cycle }} / ¥{{ number_format((float)$p->price, 2) }}）
                 </option>
             @endforeach
         </select>
         <small class="muted">订单会写入套餐快照（plan_name / billing_cycle / plan_snapshot），便于后续套餐变更时追溯。</small>
+        @if($hasLockedSubscription)
+            <small class="muted">已从订阅上下文锁定套餐（当前阶段不支持跨套餐续费；升级/降级后续再建）。</small>
+        @endif
     </label>
 
     <label>