feat: Implement redis adapter for chat-service (#74)

Description
---
This pull request primarily introduces the redis adapter to the chat
service and making adaptations to accommodate for horizontal scaling of
the service. Additionally introduces bug fixes to the chat service to
function in-line with the collab service behavior.

Author: wzhua02

backend-services/chatting-backend-service/.env.example

@@ -1 +1,2 @@
 PORT=5286
+REDIS_URL=redis://redis:6379

backend-services/chatting-backend-service/package-lock.json

@@ -14,10 +14,12 @@
     "lint": "eslint ."
   },
   "dependencies": {
+    "@socket.io/redis-adapter": "^8.3.0",
     "cors": "^2.8.5",
     "dotenv": "^17.2.3",
     "express": "^5.1.0",
     "mongoose": "^8.19.1",
+    "redis": "^5.9.0",
     "socket.io": "^4.8.1"
   },
   "devDependencies": {

backend-services/chatting-backend-service/package.json

@@ -0,0 +1,69 @@
+import { createClient } from "redis";
+
+class RedisService {
+  static instance = null;
+
+  constructor() {
+    const redisUrl = process.env.REDIS_URL || "redis://localhost:6379";
+
+    this.pubClient = createClient({ url: redisUrl });
+    this.subClient = this.pubClient.duplicate();
+    this.appClient = this.pubClient.duplicate();
+
+    const handleError = (label) => (err) =>
+      console.error(`Redis ${label} Error:`, err);
+
+    this.pubClient.on("error", handleError("Pub"));
+    this.subClient.on("error", handleError("Sub"));
+    this.appClient.on("error", handleError("App"));
+  }
+
+  async connect() {
+    await Promise.all([
+      this.pubClient.connect(),
+      this.subClient.connect(),
+      this.appClient.connect(),
+    ]);
+    console.log("Redis connected");
+  }
+
+  static async getInstance() {
+    if (!RedisService.instance) {
+      const service = new RedisService();
+      await service.connect();
+      RedisService.instance = service;
+    }
+    return RedisService.instance;
+  }
+
+  // -------- Room operations --------
+  async addOrUpdateUser(roomId, userId, data) {
+    await this.appClient.hSet(
+      `room:${roomId}:users`,
+      userId,
+      JSON.stringify(data),
+    );
+  }
+
+  async getUser(roomId, userId) {
+    const raw = await this.appClient.hGet(`room:${roomId}:users`, userId);
+    return raw ? JSON.parse(raw) : null;
+  }
+
+  async getAllUsers(roomId) {
+    const users = await this.appClient.hGetAll(`room:${roomId}:users`);
+    return Object.fromEntries(
+      Object.entries(users).map(([id, val]) => [id, JSON.parse(val)]),
+    );
+  }
+
+  async removeUser(roomId, userId) {
+    await this.appClient.hDel(`room:${roomId}:users`, userId);
+  }
+
+  async deleteRoom(roomId) {
+    await this.appClient.del(`room:${roomId}:users`);
+  }
+}
+
+export default RedisService;