feat: name live feed routes and sources for what they serve

The flight, military and vessel feeds were named after the providers the
bundled server uses (/api/opensky, /api/adsblol/mil, /api/ais-live and
their track routes; createOpenSkySource, createAdsbLolSource,
createAisStreamSource). Any server should be able to answer a feed from
its own provider, so the feeds are now:

  /api/flights   /api/flights/track?icao24=
  /api/military  /api/military/track?hex=
  /api/vessels   /api/vessels/track?mmsi=

with createFlightSource, createMilitarySource and createVesselSource. The
old routes are removed. Each server names its provider in X-Feed-Source
(the bundled server sends OpenSky Network, adsb.lol and AISStream, so its
attribution is unchanged) and the military feed's cache state and age in
X-Feed-Cache and X-Feed-Age-Ms. Sources report the provider the server
names and fall back to the feed's name.

.env.example

@@ -122,7 +122,7 @@ PORT=4173
 
 # Optional: Live AIS vessels layer
 # Browser fetch path. Keep same-origin so the private stream key stays server-side.
-VITE_AIS_LIVE_API_URL=/api/ais-live
+VITE_AIS_LIVE_API_URL=/api/vessels
 # Client render cap. The server can keep a larger AISStream cache, but drawing
 # every global vessel at once can overwhelm WebGL on laptops.
 VITE_AIS_LIVE_MAX_ROWS=12000