fix(pre-planting): getEligibility 返回 canPurchase 字段修复"待开启"问题

后端 getEligibility() 原先只返回 hasPrePlanting/canTrade 等内部字段，
缺少前端购买页期望的 canPurchase/maxAdditional/message 字段。
由于 json['canPurchase'] ?? false 默认为 false，导致购买页始终显示"待开启"。

修复：getEligibility() 现在先查询 admin config 的 isActive 状态，
结合用户持仓计算出 canPurchase/maxAdditional/message，同时保留原有字段。

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

backend/services/planting-service/src/pre-planting/application/services/pre-planting-application.service.ts

@@ -353,7 +353,11 @@ export class PrePlantingApplicationService {
   }
 
   /**
-   * 获取预种资格信息（供内部 API 使用）
+   * 获取预种资格信息（供内部 API + 移动端购买页使用）
+   *
+   * 返回字段同时满足两个消费者：
+   * - 内部 API: hasPrePlanting, canApplyAuthorization, canTrade
+   * - 移动端购买页: canPurchase, maxAdditional, message
    */
   async getEligibility(accountSequence: string): Promise<{
     hasPrePlanting: boolean;
@@ -361,9 +365,40 @@ export class PrePlantingApplicationService {
     totalTreesMerged: number;
     canApplyAuthorization: boolean;
     canTrade: boolean;
+    canPurchase: boolean;
+    maxAdditional: number | null;
+    message: string | null;
   }> {
+    const config = await this.adminClient.getPrePlantingConfig();
+
     return this.prisma.$transaction(async (tx) => {
       const position = await this.positionRepo.findByAccountSequence(tx, accountSequence);
+
+      // 计算购买资格
+      let canPurchase = true;
+      let maxAdditional: number | null = null;
+      let message: string | null = null;
+
+      if (config.isActive) {
+        // 开关打开，任何人都可以购买
+        canPurchase = true;
+      } else if (!position || position.totalPortions === 0) {
+        // 开关关闭 + 无持仓 → 不可购买
+        canPurchase = false;
+        message = '预种功能待开启';
+      } else {
+        // 开关关闭 + 有持仓 → 看是否还能续购凑满
+        const additionalNeeded = position.maxAdditionalPortionsToMerge();
+        if (additionalNeeded === 0) {
+          canPurchase = false;
+          message = '预种功能已关闭，您当前份额已满，无法继续购买';
+        } else {
+          canPurchase = true;
+          maxAdditional = additionalNeeded;
+          message = `预种功能已暂停，您可继续购买 ${additionalNeeded} 份以凑满合并`;
+        }
+      }
+
       if (!position) {
         return {
           hasPrePlanting: false,
@@ -371,6 +406,9 @@ export class PrePlantingApplicationService {
           totalTreesMerged: 0,
           canApplyAuthorization: true,
           canTrade: true,
+          canPurchase,
+          maxAdditional,
+          message,
         };
       }
       return {
@@ -379,6 +417,9 @@ export class PrePlantingApplicationService {
         totalTreesMerged: position.totalTreesMerged,
         canApplyAuthorization: position.totalTreesMerged >= 1,
         canTrade: position.totalTreesMerged >= 1,
+        canPurchase,
+        maxAdditional,
+        message,
       };
     });
   }