feat(sdr): local receivers — browser RTL-SDR, Local ADS-B layer and decoder feeds (#732)

Adds a Local ADS-B layer fed by your own receivers: a browser WebUSB
RTL-SDR (with FM listening) and local 1090/978 MHz decoder feeds
(LOCAL_RECEIVER_FEEDS, local and private addresses only). Local aircraft
draw in magenta with class, silhouettes, 3D models, trails and a position
sanity filter; the click card names the band and input that heard them.

Builds on Sameh Khamis's SDR prototype and the initial GEV port by
Mazeyar Moeini Feizabadi (#134); the decoder-feed design follows the
notes on #57. Closes #57.

Co-authored-by: Sameh Khamis <[email protected]>
Co-authored-by: Mazeyar Moeini Feizabadi <[email protected]>

DATA_SOURCES.md

@@ -65,6 +65,10 @@ How to read this:
 | **Re:Earth Terrain** (Mapterhorn)                                     | Terrain (keyless globe stacks — OSM etc. — + `/api/terrain/heights` ellipsoidal-height lookups)                                     | Terrain mesh: CC BY 4.0; geoid: EGM2008 (NGA, public domain)                                                                                                                                                                                                                                                                                          | "Terrain (keyless globe stacks): Re:Earth Terrain / Mapterhorn (CC BY 4.0) / EGM2008 (NGA)"                                                 |
 | **OSRM on the FOSSGIS routing servers** (`routing.openstreetmap.de`) | Street-following routes for the Directions layer and voice route annotations, via `/api/route` | [FOSSGIS routing usage policy](https://routing.openstreetmap.de/about.html): "Display the required attribution and display a link to 'fix the map'", "Use a valid user agent and, if applicable, a correct referrer", "One request per second max", "No scraping, no heavy usage". The full policy is the German [FOSSGIS Nutzungsbedingungen](https://www.fossgis.de/arbeitsgruppen/osm-server/nutzungsbedingungen/); FOSSGIS also states that the server may be embedded in your own pages but "eine gewerbliche Nutzung ist nur mit Einschränkungen erlaubt" (commercial use only with restrictions). Route data derives from OpenStreetMap (ODbL 1.0) | "Routing: OSRM on the FOSSGIS servers" + "© OpenStreetMap contributors" + a "fix the map" link — shown in the Data attribution popover |
 
+### Local receiver input (not fetched from a service)
+
+The optional Local ADS-B layer and the broadcast-FM receiver use radio you receive yourself: a USB RTL-SDR opened in the browser through WebUSB, or the `aircraft.json` of a decoder you run on your own machine or network (`LOCAL_RECEIVER_FEEDS`). Radio samples, audio and the optional receiver location stay in the browser. Decoder feeds are read by your local GEV server and relayed only to your browser, with about a second of in-memory caching. When you select a local aircraft, its ICAO address (and an airline-style callsign) is looked up, and aircraft that qualify for a 3D model are looked up by ICAO address, through the same adsbdb proxy the public flight layers use; your server caches those answers on disk and treats them as fresh for 24 hours. Received radio data is not stored or redistributed. ADS-B is an unauthenticated public broadcast and may be incomplete, delayed, spoofed or decoded in error; it is not suitable for navigation or operational use.
+
 ### Notes on the live sources
 
 - **Google Maps Platform.** You supply your own API key and are bound by [Google's ToS](https://cloud.google.com/maps-platform/terms). Google Maps Content (tiles, geocodes, places) **may not be cached, stored, rehosted, or committed** — this app only ever uses it live, which is the compliant pattern. The "Google" attribution is displayed on the globe and must stay visible. Restrict your key (see [SECURITY.md](SECURITY.md)).