feat: 初始化 Nest 服务骨架与多平台 Chat SSE 网关

- 新增 NestJS + Fastify 入口、配置模块与 Swagger 集成
- 划分 client-app / admin-app 与 shared-domain ai-gateway
- 实现统一 SSE Chat 接口，支持千问、DeepSeek、火山引擎非流式上游与网关分片输出
- 补充项目方案与 JWT 最小实现文档

Made-with: Cursor

src/apps/client-app/chat/controllers/chat.controller.ts

@@ -0,0 +1,52 @@
+import { Body, Controller, Inject, Post, Res } from '@nestjs/common';
+import { FastifyReply } from 'fastify';
+import { StreamChatRequest } from '@shared/ai-gateway/types/chat.types';
+import { ProviderRouterService } from '@shared/ai-gateway/router/provider-router.service';
+
+function formatSse(event: string, data: unknown) {
+  return `event: ${event}\ndata: ${JSON.stringify(data)}\n\n`;
+}
+
+function sleep(ms: number) {
+  return new Promise((resolve) => setTimeout(resolve, ms));
+}
+
+@Controller('client/v1/chat')
+export class ChatController {
+  constructor(
+    @Inject(ProviderRouterService)
+    private readonly router: ProviderRouterService,
+  ) {}
+
+  @Post('completions/stream')
+  async streamChat(
+    @Body() body: StreamChatRequest,
+    @Res() reply: FastifyReply,
+  ) {
+    const response = await this.router.routeAndStream(body);
+
+    reply.raw.setHeader('Content-Type', 'text/event-stream; charset=utf-8');
+    reply.raw.setHeader('Cache-Control', 'no-cache, no-transform');
+    reply.raw.setHeader('Connection', 'keep-alive');
+    reply.raw.setHeader('X-Accel-Buffering', 'no');
+    reply.raw.flushHeaders?.();
+
+    reply.raw.write(
+      formatSse('meta', {
+        requestId: response.requestId,
+        platform: response.providerCode,
+        model: response.model,
+      }),
+    );
+
+    for (const chunk of response.chunks) {
+      reply.raw.write(formatSse('delta', { delta: chunk.content }));
+      await sleep(120);
+    }
+
+    reply.raw.write(formatSse('usage', response.usage));
+    reply.raw.write(formatSse('done', { finishReason: 'stop' }));
+    reply.raw.end();
+  }
+}
+