feat: 更新AI教练控制器，增加用户聊天次数管理功能

- 在AI教练控制器中引入用户聊天次数的检查，确保用户在进行对话前有足够的聊天次数。
- 新增用户服务方法以获取和扣减用户的聊天次数，优化用户体验。
- 调整默认免费聊天次数为5次，提升系统的使用限制管理。

src/ai-coach/ai-coach.controller.ts

@@ -1,4 +1,4 @@
-import { Body, Controller, Delete, Get, Param, Post, Query, Res, StreamableFile, UseGuards } from '@nestjs/common';
+import { Body, Controller, Delete, Get, HttpException, HttpStatus, Logger, Param, Post, Query, Res, StreamableFile, UseGuards } from '@nestjs/common';
 import { ApiTags, ApiOperation, ApiBody, ApiQuery, ApiParam } from '@nestjs/swagger';
 import { Response } from 'express';
 import { JwtAuthGuard } from '../common/guards/jwt-auth.guard';
@@ -7,12 +7,17 @@ import { AccessTokenPayload } from '../users/services/apple-auth.service';
 import { AiCoachService } from './ai-coach.service';
 import { AiChatRequestDto, AiChatResponseDto, AiResponseDataDto } from './dto/ai-chat.dto';
 import { PostureAssessmentRequestDto, PostureAssessmentResponseDto } from './dto/posture-assessment.dto';
+import { UsersService } from '../users/users.service';
 
 @ApiTags('ai-coach')
 @Controller('ai-coach')
 @UseGuards(JwtAuthGuard)
 export class AiCoachController {
-  constructor(private readonly aiCoachService: AiCoachService) { }
+  private readonly logger = new Logger(AiCoachController.name);
+  constructor(
+    private readonly aiCoachService: AiCoachService,
+    private readonly usersService: UsersService,
+  ) { }
 
   @Post('chat')
   @ApiOperation({ summary: '流式大模型对话（普拉提教练）' })
@@ -26,6 +31,13 @@ export class AiCoachController {
     const stream = body.stream !== false; // 默认流式
     const userContent = body.messages?.[body.messages.length - 1]?.content || '';
 
+    // 判断用户是否有聊天次数
+    const usageCount = await this.usersService.getUserUsageCount(userId);
+    if (usageCount <= 0) {
+      this.logger.error(`chat: ${userId} has no usage count`);
+      throw new HttpException('用户没有聊天次数', HttpStatus.FORBIDDEN);
+    }
+
     // 创建或沿用会话ID，并保存用户消息
     const { conversationId } = await this.aiCoachService.createOrAppendMessages({
       userId,
@@ -45,6 +57,8 @@ export class AiCoachController {
       confirmationData: body.confirmationData,
     });
 
+    await this.usersService.deductUserUsageCount(userId);
+
     // 检查是否返回结构化数据（如确认选项）
     // 结构化数据必须使用非流式模式返回
     if (typeof result === 'object' && 'type' in result) {