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,