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/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
+    }
+  }
+}