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.

server/providers/aircraft/opensky.js

@@ -380,8 +380,12 @@ function openSkySourceIsStale(sourceEpochMs, now = Date.now()) {
  */
 export function openSkyProxy() {
   const installMiddleware = (server) => {
-    server.middlewares.use('/api/opensky', async (req, res) => {
+    server.middlewares.use('/api/flights', async (req, res, next) => {
       try {
+        // This mount also matches its /track route, served by its own handler.
+        if (new URL(req.url || '/', 'http://localhost').pathname !== '/')
+          return next();
+        res.setHeader('X-Feed-Source', 'OpenSky Network');
         const requestedMode = normalizeOpenSkyAuthMode(
           process.env.OPENSKY_AUTH_MODE,
         );