feat(admin): 预种管理页添加"待合并总量"统计项

- planting-service stats 端点新增 pendingMergePortions 字段
  从 PrePlantingPosition 表聚合 SUM(available_portions)
- admin-web 统计卡片区新增"待合并总量(份)"显示
- 网格布局从 4 列调整为 5 列

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

backend/services/planting-service/src/pre-planting/api/controllers/internal-pre-planting.controller.ts

@@ -244,6 +244,7 @@ export class InternalPrePlantingController {
       treesMergedAgg,
       totalUsers,
       pendingContracts,
+      pendingPortionsAgg,
     ] = await Promise.all([
       this.prisma.prePlantingOrder.count(),
       this.prisma.prePlantingOrder.aggregate({ _sum: { portionCount: true } }),
@@ -255,6 +256,7 @@ export class InternalPrePlantingController {
       this.prisma.prePlantingMerge.aggregate({ _sum: { treeCount: true } }),
       this.prisma.prePlantingPosition.count(),
       this.prisma.prePlantingMerge.count({ where: { contractStatus: 'PENDING' } }),
+      this.prisma.prePlantingPosition.aggregate({ _sum: { availablePortions: true } }),
     ]);
 
     return {
@@ -265,6 +267,7 @@ export class InternalPrePlantingController {
       totalTreesMerged: treesMergedAgg._sum.treeCount ?? 0,
       totalUsers,
       pendingContracts,
+      pendingMergePortions: pendingPortionsAgg._sum.availablePortions ?? 0,
     };
   }
 

frontend/admin-web/src/app/(dashboard)/pre-planting/page.tsx

@@ -280,6 +280,12 @@ export default function PrePlantingPage() {
             </div>
             <div className={styles.prePlanting__statLabel}>总金额 (绿积分)</div>
           </div>
+          <div className={styles.prePlanting__statCard}>
+            <div className={styles.prePlanting__statValue}>
+              {statsLoading ? '-' : formatNumber(stats?.pendingMergePortions ?? 0)}
+            </div>
+            <div className={styles.prePlanting__statLabel}>待合并总量(份)</div>
+          </div>
           <div className={styles.prePlanting__statCard}>
             <div className={styles.prePlanting__statValue}>
               {statsLoading ? '-' : formatNumber(stats?.totalTreesMerged ?? 0)}

frontend/admin-web/src/app/(dashboard)/pre-planting/pre-planting.module.scss

@@ -154,7 +154,7 @@
   // 统计卡片网格
   &__statsGrid {
     display: grid;
-    grid-template-columns: repeat(4, 1fr);
+    grid-template-columns: repeat(5, 1fr);
     gap: 16px;
 
     @media (max-width: 1200px) {