fix: RagChatModule 导入 AiModule，修复 AiGatewayService 未注入

AiGatewayService 使用了 @Optional() 导致不报错但始终为 null，
sendMessage 永远走 fallbackReply。现在导入 AiModule 正确注入。

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/modules/rag-chat/rag-chat.module.ts

@@ -3,8 +3,10 @@ import { RagChatController } from './rag-chat.controller';
 import { AdminRagChatController } from './admin-rag-chat.controller';
 import { RagChatService } from './rag-chat.service';
 import { PrismaService } from '../../infrastructure/database/prisma.service';
+import { AiModule } from '../ai/ai.module';
 
 @Module({
+  imports: [AiModule],
   controllers: [RagChatController, AdminRagChatController],
   providers: [RagChatService, PrismaService],
   exports: [RagChatService],