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/__init__.py

@@ -32,6 +32,7 @@ from .hackrf import HackRFCommandBuilder
 from .limesdr import LimeSDRCommandBuilder
 from .rtlsdr import RTLSDRCommandBuilder
 from .sdrplay import SDRPlayCommandBuilder
+from .usrp import USRPCommandBuilder
 from .validation import (
     SDRValidationError,
     get_capabilities_for_type,
@@ -53,6 +54,7 @@ class SDRFactory:
         SDRType.HACKRF: HackRFCommandBuilder,
         SDRType.AIRSPY: AirspyCommandBuilder,
         SDRType.SDRPLAY: SDRPlayCommandBuilder,
+        SDRType.USRP: USRPCommandBuilder,
     }
 
     @classmethod
@@ -211,6 +213,7 @@ __all__ = [
     "HackRFCommandBuilder",
     "AirspyCommandBuilder",
     "SDRPlayCommandBuilder",
+    "USRPCommandBuilder",
     # Validation
     "SDRValidationError",
     "validate_frequency",