fix: pin paho-mqtt to v1.x (v2 broke callback API)

paho-mqtt v2 changed Client constructor and on_connect callback
signatures, breaking the Meshtastic MQTT bridge. Pin to <2.0.0
so the existing v1 code works correctly in Docker.

backend/pyproject.toml

@@ -21,7 +21,7 @@ dependencies = [
     "reverse-geocoder==1.5.1",
     "sgp4==2.23",
     "orjson>=3.10.0",
-    "paho-mqtt>=1.6.0",
+    "paho-mqtt>=1.6.0,<2.0.0",
     "slowapi==0.1.9",
     "uvicorn==0.34.0",
     "yfinance==0.2.54",