docs(changelog): note OpenSky snapshot compression (#795)

CHANGELOG.md

@@ -1,5 +1,13 @@
 # Changelog
 
+- The live flight snapshot is compressed on the wire. `/api/opensky` gzips its
+  cached global snapshot and its regional adsb.lol fallback once per upstream
+  refresh and serves that frame to clients offering gzip — roughly 860 KB
+  becomes roughly 258 KB — while a client that offers nothing receives the same
+  bytes uncompressed. Both answers carry `Vary: Accept-Encoding`, and a response
+  the proxy does not cache, such as an auth failure, is never sent under a
+  cached snapshot's encoding (daikaginza, #795).
+
 - Public Overpass instances are no longer used by default. Street Traffic
   roads come from TomTom flow tiles, OpenFreeMap vector tiles, or both, chosen
   on the layer row (TomTom / OSM / Hybrid) or with `?trafficRoads=`. With a