fix(server): declare Allow: GET on the new 405 responses

RFC 9110 section 15.5.6 requires a 405 to name the methods it accepts, and
/api/radio/stations already does this in-tree. Thanks to kvnloo for catching it
on review. The same gap on the pre-existing handlers is tracked in #809 and is
deliberately left alone here.

server/providers/aircraft/opensky.js

@@ -354,7 +354,10 @@ export function openSkyProxy() {
       // /api/geocode and the other query proxies state it, rather than
       // answering any verb with the same body and ignoring the request body.
       if (req.method !== 'GET') {
-        res.writeHead(405, { 'Content-Type': 'application/json' });
+        res.writeHead(405, {
+          'Content-Type': 'application/json',
+          Allow: 'GET',
+        });
         res.end(JSON.stringify({ error: 'Method Not Allowed' }));
         return;
       }