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/mqtt.py

@@ -0,0 +1,107 @@
+"""
+Optional MQTT data export.
+
+Publishes decoded events from all modules to a configurable MQTT broker.
+Disabled when INTERCEPT_MQTT_BROKER is not set — no broker, no connection,
+no error.
+
+Topics follow the pattern:
+    <prefix>/<module>/<event_type>
+
+Payload is JSON, matching the SSE event structure already used by the
+frontend.  Configure via environment variables:
+
+    INTERCEPT_MQTT_BROKER        broker hostname / IP  (required to enable)
+    INTERCEPT_MQTT_PORT          port (default 1883)
+    INTERCEPT_MQTT_USER          username (optional)
+    INTERCEPT_MQTT_PASSWORD      password (optional)
+    INTERCEPT_MQTT_TOPIC_PREFIX  topic prefix (default "intercept")
+    INTERCEPT_MQTT_RETAIN        retain flag (default false)
+"""
+
+from __future__ import annotations
+
+import json
+import logging
+import threading
+from typing import Any
+
+logger = logging.getLogger(__name__)
+
+_client = None
+_lock = threading.Lock()
+_enabled: bool | None = None  # None = not yet initialised
+
+
+def _get_config():
+    import config
+
+    return config
+
+
+def _is_enabled() -> bool:
+    global _enabled
+    if _enabled is None:
+        cfg = _get_config()
+        _enabled = bool(cfg.MQTT_BROKER)
+    return _enabled
+
+
+def _get_client():
+    """Return a connected paho MQTT client, creating it on first call."""
+    global _client
+    if _client is not None:
+        return _client
+
+    with _lock:
+        if _client is not None:
+            return _client
+
+        try:
+            import paho.mqtt.client as mqtt
+        except ImportError:
+            logger.warning("paho-mqtt not installed; MQTT export disabled. Install with: pip install paho-mqtt")
+            return None
+
+        cfg = _get_config()
+
+        client = mqtt.Client(client_id="intercept", clean_session=True)
+        if cfg.MQTT_USER:
+            client.username_pw_set(cfg.MQTT_USER, cfg.MQTT_PASSWORD or None)
+
+        try:
+            client.connect(cfg.MQTT_BROKER, cfg.MQTT_PORT, keepalive=60)
+            client.loop_start()
+            logger.info("MQTT connected to %s:%d (prefix=%s)", cfg.MQTT_BROKER, cfg.MQTT_PORT, cfg.MQTT_TOPIC_PREFIX)
+            _client = client
+        except Exception as exc:
+            logger.warning("MQTT connect failed: %s — export disabled until restart", exc)
+            return None
+
+    return _client
+
+
+def publish(mode: str, event: dict[str, Any], event_type: str | None = None) -> None:
+    """Publish a decoded event to the MQTT broker.
+
+    Args:
+        mode:       Source module name (e.g. 'aprs', 'adsb', 'pager').
+        event:      The decoded event dict.
+        event_type: Optional sub-type string (e.g. 'packet', 'aircraft').
+    """
+    if not _is_enabled():
+        return
+
+    client = _get_client()
+    if client is None:
+        return
+
+    cfg = _get_config()
+    subtopic = event_type or mode
+    topic = f"{cfg.MQTT_TOPIC_PREFIX}/{mode}/{subtopic}"
+
+    try:
+        payload = json.dumps(event, default=str)
+        client.publish(topic, payload, retain=cfg.MQTT_RETAIN)
+    except Exception as exc:
+        logger.debug("MQTT publish error on %s: %s", topic, exc)