feat: APRS export, USRP support, and MQTT data export (closes #222)

- GET /aprs/export?format=json|csv downloads all tracked APRS stations
- USRP (Ettus N200/B200/B210) detected via SoapyUHD and supported across
  all signal modes (FM demod, ADS-B, ISM, AIS, I/Q capture)
- Optional MQTT publisher broadcasts decoded events from every module;
  enabled by setting INTERCEPT_MQTT_BROKER, disabled by default
- paho-mqtt added to requirements.txt (optional dep)
- .gitignore: add *.kismet, aircraft_db.json/meta, SoapySDR/
- Bump version to 2.30.0

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -60,6 +60,17 @@ data/radiosonde/
 # SDR capture files (large IQ recordings)
 data/subghz/captures/
 
+# Kismet capture files
+*.kismet
+*.kismet.json
+
+# ADS-B aircraft database (downloaded at runtime)
+aircraft_db.json
+aircraft_db_meta.json
+
+# SoapySDR build directory (compiled during setup, contains its own .git)
+SoapySDR/
+
 # Env files
 .env
 .env.*