Local ADS-B for airband context (PREPARED, NOT ENABLED)

Why: public aggregators (adsb.lol, adsb.fi, OpenSky; checked 2026-10-20) return 0 aircraft over
St. Petersburg/Moscow while London returns dozens -- there is no public coverage of Russian airspace,
so the "в эфире рядом" panel stays empty until a local 1090 MHz receiver exists.

Steps when the dongle arrives:
 1. rtl_eeprom -d <index> -s 1090 ; replug
 2. sudo apt install readsb ; sudo cp /opt/pi/sdr/readsb/readsb.default /etc/default/readsb
 3. sudo systemctl enable --now readsb ; check /run/readsb/aircraft.json
 4. Nothing else: sdr-classify picks up /run/readsb/aircraft.json automatically (source "readsb").
 Receiver position for distances: RX_LAT/RX_LON in /opt/pi/sdr/classify/classify.py (@@LAT@@ N, @@LON@@ E, unconfirmed).
 Antenna: a 1090 MHz ground plane / collinear as high as possible; HackRF must not be used for this.
