fix: add meshtastic, PyNaCl, vaderSentiment to dependencies

Full import audit found these packages used but missing from
pyproject.toml — all silently broken in Docker:
- meshtastic: MQTT protobuf decode (why US/LongFast chat was empty)
- PyNaCl: DM sealed-box encryption
- vaderSentiment: oracle sentiment analysis (unguarded, would crash)

backend/pyproject.toml

@@ -20,9 +20,12 @@ dependencies = [
     "requests==2.31.0",
     "reverse-geocoder==1.5.1",
     "sgp4==2.23",
+    "meshtastic>=2.5.0",
     "orjson>=3.10.0",
     "paho-mqtt>=1.6.0,<2.0.0",
+    "PyNaCl>=1.5.0",
     "slowapi==0.1.9",
+    "vaderSentiment>=3.3.0",
     "uvicorn==0.34.0",
     "yfinance==0.2.54",
 ]