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>

utils/sdr/detection.py

@@ -53,6 +53,7 @@ def _get_capabilities_for_type(sdr_type: SDRType) -> SDRCapabilities:
     from .limesdr import LimeSDRCommandBuilder
     from .rtlsdr import RTLSDRCommandBuilder
     from .sdrplay import SDRPlayCommandBuilder
+    from .usrp import USRPCommandBuilder
 
     builders = {
         SDRType.RTL_SDR: RTLSDRCommandBuilder,
@@ -60,6 +61,7 @@ def _get_capabilities_for_type(sdr_type: SDRType) -> SDRCapabilities:
         SDRType.HACKRF: HackRFCommandBuilder,
         SDRType.AIRSPY: AirspyCommandBuilder,
         SDRType.SDRPLAY: SDRPlayCommandBuilder,
+        SDRType.USRP: USRPCommandBuilder,
     }
 
     builder_class = builders.get(sdr_type)
@@ -90,8 +92,8 @@ def _driver_to_sdr_type(driver: str) -> SDRType | None:
         "airspy": SDRType.AIRSPY,
         "airspyhf": SDRType.AIRSPY,  # Airspy HF+ uses same builder
         "sdrplay": SDRType.SDRPLAY,
+        "uhd": SDRType.USRP,
         # Future support
-        # 'uhd': SDRType.USRP,
         # 'bladerf': SDRType.BLADE_RF,
     }
     return mapping.get(driver.lower())