Add data layer with MongoDB and JSON fallback store

Introduce database config, MongoDB and JSON store implementations,
and a datastore factory that connects to MongoDB by default and falls
back to a local JSON file when MongoDB is unavailable.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Backend/lib/stores/mongoStore.js

@@ -0,0 +1,217 @@
+const config = require('../../config/database')
+
+
+let MongoClient
+try {
+  ;({ MongoClient } = require('mongodb'))
+} catch (error) {
+  MongoClient = null
+}
+
+
+const collectionNames = {
+  users: 'users',
+  messages: 'messages',
+}
+
+function normalizeUser(doc) {
+  if (!doc) return null
+  const {
+    _id,
+    username,
+    room,
+    isOnline,
+    joinedAt,
+    lastActiveAt,
+  } = doc
+  return {
+    id: _id,
+    username,
+    room,
+    isOnline,
+    joinedAt,
+    lastActiveAt,
+  }
+}
+
+function normalizeMessage(doc) {
+  if (!doc) return null
+  const {
+    _id,
+    room,
+    userId,
+    username,
+    text,
+    createdAt,
+  } = doc
+  return {
+    id: _id,
+    room,
+    userId,
+    username,
+    text,
+    createdAt,
+  }
+}
+
+function extractDocument(result) {
+  if (!result) return null
+  return Object.prototype.hasOwnProperty.call(result, 'value')
+    ? result.value
+    : result
+}
+
+async function createMongoStore() {
+  // Guard: fail fast with an actionable message if the driver is missing.
+  if (!MongoClient) {
+    const message =
+      'MongoDB driver is not installed. Run `npm install mongodb` in Backend/.'
+    const error = new Error(message)
+    error.code = 'MODULE_NOT_INSTALLED'
+    throw error
+  }
+
+  const client = new MongoClient(config.uri, {
+    ...config.options,
+    appName: 'chat-app',
+  })
+
+  await client.connect()
+  const db = client.db(config.dbName)
+
+  const users = db.collection(collectionNames.users)
+  const messages = db.collection(collectionNames.messages)
+
+  await Promise.all([
+    users.createIndex({ room: 1, isOnline: -1, lastActiveAt: -1 }),
+    messages.createIndex({ room: 1, createdAt: 1 }),
+  ])
+
+  console.log(
+    `Connected to MongoDB at ${
+      config.uri
+    }/${config.dbName} (collections: ${Object.values(collectionNames).join(', ')})`,
+  )
+
+  return {
+    async upsertUser({ id, username, room }) {
+      const now = new Date()
+      const result = await users.findOneAndUpdate(
+        { _id: id },
+        {
+          $set: {
+            username: username.trim(),
+            room,
+            isOnline: true,
+            lastActiveAt: now,
+            updatedAt: now,
+          },
+          $setOnInsert: {
+            joinedAt: now,
+          },
+        },
+        {
+          upsert: true,
+          returnDocument: 'after',
+        },
+      )
+      const document = extractDocument(result)
+      if (document) {
+        return normalizeUser(document)
+      }
+
+
+      const inserted = await users.findOne({ _id: id })
+      return normalizeUser(inserted)
+    },
+
+    async setUserOnline(userId, room) {
+      const result = await users.findOneAndUpdate(
+        { _id: userId },
+        {
+          $set: {
+            isOnline: true,
+            room,
+            lastActiveAt: new Date(),
+          },
+        },
+        { returnDocument: 'after' },
+      )
+      return normalizeUser(extractDocument(result))
+    },
+
+    async setUserOffline(userId) {
+      const result = await users.findOneAndUpdate(
+        { _id: userId },
+        {
+          $set: {
+            isOnline: false,
+            lastActiveAt: new Date(),
+          },
+        },
+        { returnDocument: 'after' },
+      )
+      return normalizeUser(extractDocument(result))
+    },
+
+    async updateUsername(userId, username) {
+      const result = await users.findOneAndUpdate(
+        { _id: userId },
+        {
+          $set: {
+            username: username.trim(),
+            lastActiveAt: new Date(),
+          },
+        },
+        { returnDocument: 'after' },
+      )
+      return normalizeUser(extractDocument(result))
+    },
+
+    async removeUser(userId) {
+      const result = await users.findOneAndDelete({ _id: userId })
+      return normalizeUser(extractDocument(result))
+    },
+
+    async getUser(userId) {
+      const doc = await users.findOne({ _id: userId })
+      return normalizeUser(doc)
+    },
+
+    async getParticipants(room) {
+      const docs = await users
+        .find({ room })
+        .sort({ isOnline: -1, lastActiveAt: -1 })
+        .toArray()
+      return docs.map(normalizeUser)
+    },
+
+    async addMessage({ id, room, userId, username, text }) {
+      const message = {
+        _id: id,
+        room,
+        userId,
+        username,
+        text,
+        createdAt: new Date(),
+      }
+      await messages.insertOne(message)
+      return normalizeMessage(message)
+    },
+
+    async getMessages(room, limit = 100) {
+      const docs = await messages
+        .find({ room })
+        .sort({ createdAt: 1 })
+        .limit(limit)
+        .toArray()
+      return docs.map(normalizeMessage)
+    },
+
+    async disconnect() {
+      await client.close()
+    },
+  }
+}
+
+module.exports = createMongoStore