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/datastore.js

@@ -0,0 +1,34 @@
+const path = require('path')
+const JsonStore = require('./stores/jsonStore')
+
+async function createDataStore() {
+  const requestedStore = process.env.DATA_STORE
+  const forceJson = requestedStore === 'json'
+  const strictMongo = requestedStore === 'mongo'
+
+  if (forceJson) {
+    const filePath = path.join(__dirname, '..', 'data', 'db.json')
+    console.log(`Using JSON fallback store at ${filePath}`)
+    return new JsonStore(filePath)
+  }
+
+  try {
+    const createMongoStore = require('./stores/mongoStore')
+    return await createMongoStore()
+  } catch (error) {
+    if (strictMongo) {
+      throw error
+    }
+
+    console.warn(
+      'MongoDB store unavailable, falling back to JSON store.',
+      error.message,
+    )
+  }
+
+  const filePath = path.join(__dirname, '..', 'data', 'db.json')
+  console.log(`Using JSON fallback store at ${filePath}`)
+  return new JsonStore(filePath)
+}
+
+module.exports = createDataStore