Commit missing MQTT/history/scheduler files referenced by meters.js

routes/meters.js already imported lib/mqtt.js and lib/history.js in an
earlier uncommitted change; a prior commit staged the whole meters.js
file (picking up those imports) without staging the modules themselves,
crashing production with ERR_MODULE_NOT_FOUND. Completing the commit:
package.json (mqtt dependency), index.js (wires connectMqtt/startScheduler,
both already error-guarded if the broker isn't reachable), and the three
lib files themselves.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/package.json

@@ -13,6 +13,7 @@
     "@fastify/static": "^7.0.4",
     "fastify": "^4.28.1",
     "jose": "^5.9.6",
+    "mqtt": "^5.10.3",
     "pg": "^8.13.1"
   }
 }