fix: gravity deducted before AI call, causing deduction on failure

- Split checkAndDeductInterview into checkInterview (read-only) and
  deductInterview (deduction only)
- Restructure interview.create(): check gravity -> AI call -> deduct on success
- If AI call fails, gravity is never deducted
- Keep old checkAndDeductInterview for backward compatibility

backend/src/modules/interview/interview.service.ts

@@ -21,14 +21,19 @@ export class InterviewService {
   ) {}
 
   async create(userId: string, position: string) {
-    await this.quotaService.checkAndDeductInterview(userId)
+    // Step 1: 先检查引力值是否足够（不扣除）
+    const cost = await this.quotaService.checkInterview(userId)
 
+    // Step 2: AI 生成第一个问题
     const firstQuestion = await this.aiService.call({
       systemPrompt: `你是一位专业的${position}面试官。请针对校招该岗位提出第一个面试问题，要求具体且有针对性。直接输出问题，不要多余内容。`,
       userMessage: `请为${position}岗位的校招候选人提出第一个面试问题。`,
       temperature: 0.8,
     })
 
+    // Step 3: AI 调用成功后，扣除引力值并创建面试记录
+    await this.quotaService.deductInterview(userId, cost)
+
     const interview = await this.interviewModel.create({
       userId,
       position,

backend/src/modules/user/quota.service.ts

@@ -15,6 +15,36 @@ export class QuotaService {
     private pricingService: PricingService,
   ) {}
 
+  /** 仅检查面试引力值是否充足，不扣除（用于先 AI 后扣款模式） */
+  async checkInterview(userId: string): Promise<number> {
+    const user = await this.userModel.findById(userId).exec()
+    if (!user) throw new HttpException('用户不存在', HttpStatus.NOT_FOUND)
+
+    // 迁移旧字段到 gravity
+    if ((user.gravity ?? 0) <= 0 && this.hasOldCredits(user)) {
+      await this.migrateOldCredits(userId)
+    }
+
+    const rates = (await this.pricingService.getConfig()).gravityRates
+    const cost = rates.interviewPerUse
+
+    // 检查 gravity 或 shareCredits 是否足够
+    const gravOk = (user.gravity ?? 0) >= cost
+    const shareOk = (user.shareCredits ?? 0) > 0
+    if (!gravOk && !shareOk) {
+      throw new HttpException('引力值不足，请充值或分享获取', HttpStatus.FORBIDDEN)
+    }
+    return cost
+  }
+
+  /** 扣除面试引力值（在 AI 调用成功后调用） */
+  async deductInterview(userId: string, cost: number) {
+    const result = await this.deductGravityOrFallback(userId, cost)
+    if (!result) {
+      throw new HttpException('引力值不足', HttpStatus.FORBIDDEN)
+    }
+  }
+
   /** 检查并扣除面试引力值（所有计划统一走引力值） */
   async checkAndDeductInterview(userId: string) {
     const user = await this.userModel.findById(userId).exec()