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.

src/tooling/previewServing.test.mjs

@@ -97,8 +97,8 @@ test('real dev and built-preview servers serve provider JSON and terminate unkno
     const origin = `http://127.0.0.1:${server.httpServer.address().port}`;
     try {
       for (const [route, status] of [
-        ['/api/opensky', 200],
-        ['/api/adsblol/mil', 200],
+        ['/api/flights', 200],
+        ['/api/military', 200],
         ['/api/adsbdb/type/invalid', 400],
         ['/api/firms/status', 200],
         ['/api/fire-perimeters/inciweb/publication/invalid', 400],