feat(export): AIS UDP NMEA forward and JSON export endpoints for AIS/ADS-B

AIS:
- New optional NMEA UDP forwarding via AIS-catcher's -u flag, configurable
  from the AIS sidebar (host + port). Lets OpenCPN and other NMEA tools
  receive live vessel data directly. All SDR builders updated.
- New GET /ais/vessels endpoint — clean JSON snapshot of tracked vessels
  for REST integration

ADS-B:
- New GET /adsb/aircraft endpoint — JSON snapshot of all tracked aircraft,
  with optional ?icao= and ?military=true filters. Response includes a
  reminder that port 30003 (SBS) is already available for tools like
  Virtual Radar Server and OpenCPN's AIS/target plugin.

Closes #90

utils/sdr/base.py

@@ -165,7 +165,9 @@ class CommandBuilder(ABC):
         device: SDRDevice,
         gain: float | None = None,
         bias_t: bool = False,
-        tcp_port: int = 10110
+        tcp_port: int = 10110,
+        udp_host: str | None = None,
+        udp_port: int | None = None,
     ) -> list[str]:
         """
         Build AIS decoder command for vessel tracking.
@@ -175,6 +177,8 @@ class CommandBuilder(ABC):
             gain: Gain in dB (None for auto)
             bias_t: Enable bias-T power (for active antennas)
             tcp_port: TCP port for JSON output server
+            udp_host: Optional host to forward NMEA 0183 sentences via UDP
+            udp_port: UDP port for NMEA forwarding (required if udp_host set)
 
         Returns:
             Command as list of strings for subprocess