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/data/flights.test.mjs

@@ -203,7 +203,7 @@ test('flights poll refreshes tracked callsign/FL/kts and marks a missed poll STA
   const nowSec = Math.floor(Date.now() / 1000);
   let openskyPoll = 0;
   globalThis.fetch = async (url) => {
-    if (!String(url).startsWith('/api/opensky')) {
+    if (!String(url).startsWith('/api/flights')) {
       return { ok: true, status: 200, json: async () => ({ ac: [] }) };
     }
     const states = openskyPoll++ === 0