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/config/database.js

@@ -0,0 +1,25 @@
+const boolFromEnv = (value, fallback = false) => {
+  if (value === undefined || value === '') return fallback
+  return ['1', 'true', 'yes', 'on'].includes(value.toLowerCase())
+}
+
+const numberFromEnv = (value, fallback) => {
+  if (value === undefined || value === '') return fallback
+  const parsed = Number(value)
+  return Number.isFinite(parsed) ? parsed : fallback
+}
+
+const config = {
+  uri: process.env.MONGO_URI || 'mongodb://127.0.0.1:27017',
+  dbName: process.env.MONGO_DB || 'chatapp',
+  options: {
+    user: process.env.MONGO_USER || undefined,
+    pass: process.env.MONGO_PASSWORD || undefined,
+    socketTimeoutMS: numberFromEnv(process.env.MONGO_SOCKET_TIMEOUT, 45000),
+    connectTimeoutMS: numberFromEnv(process.env.MONGO_CONNECT_TIMEOUT, 30000),
+    replicaSet: process.env.MONGO_REPLICA_SET || undefined,
+    tls: boolFromEnv(process.env.MONGO_TLS, false),
+  },
+}
+
+module.exports = config

Backend/config/index.js

@@ -0,0 +1,29 @@
+const fs = require('fs')
+const path = require('path')
+
+loadLocalEnv()
+
+module.exports = {
+  port: process.env.PORT || 4000,
+  clientOrigin: process.env.CLIENT_ORIGIN || '*',
+}
+
+function loadLocalEnv() {
+  const envPath = path.join(__dirname, '..', '.env')
+  if (!fs.existsSync(envPath)) {
+    return
+  }
+
+  const content = fs.readFileSync(envPath, 'utf-8')
+  for (const line of content.split(/\r?\n/)) {
+    if (!line || line.trim().startsWith('#')) continue
+
+    const [key, ...rest] = line.split('=')
+    if (!key) continue
+
+    const value = rest.join('=').trim()
+    if (key && value && process.env[key] === undefined) {
+      process.env[key] = value
+    }
+  }
+}