feat(server): compress the cached OpenSky snapshot for clients that accept gzip

The ~860 KB snapshot is held for an adaptive 9-300 s TTL and was served as
identity bytes to every request; no response in server/ set Content-Encoding or
Vary. Gzip it once per refresh beside the cached body, off the event loop, and
return the body and the header that names it together so the two can never
disagree. Live against an anonymous snapshot: 874,002 B to 258,012 B, 3.39x,
decompressing byte-identical to the uncompressed response.

server/providers/aircraft/opensky.js

@@ -1,4 +1,5 @@
 import { normalizeAdsbLolPointResponse } from '../../../src/data/adsbLolFallback.js';
+import { encodedBody, gzipBody } from '../common/compress.js';
 import {
   coalesceProxyRequest,
   readResponseJsonCapped,
@@ -15,6 +16,12 @@ let _openskyTokenExpiry = 0;
 let _openskyTokenPromise = null;
 /** @type {string|null} Cached upstream response body (JSON text). */
 let _openskyCacheBody = null;
+/**
+ * @type {Buffer|null} Gzip of `_openskyCacheBody`, written in the same step as
+ * the body it encodes so the two can never describe different snapshots. Null
+ * whenever the cached body is too small to be worth a frame.
+ */
+let _openskyCacheGzip = null;
 /** @type {number} HTTP status of the cached response. */
 let _openskyCacheStatus = 0;
 /** @type {number} Epoch-ms when the response was cached. */
@@ -271,8 +278,13 @@ async function fetchAdsbLolPointFallback(req) {
           ADSBLOL_POINT_MAX_RESPONSE_BYTES,
         );
         const normalized = normalizeAdsbLolPointResponse(payload);
+        const body = JSON.stringify(normalized);
         const record = {
-          body: JSON.stringify(normalized),
+          body,
+          // Compressed with the body it belongs to, so a cache entry served
+          // many times pays for the frame once. Entries are bounded by
+          // ADSBLOL_POINT_CACHE_MAX, so the extra residency is bounded too.
+          gzip: await gzipBody(body),
           cachedAt: Date.now(),
           count: normalized.states.length,
         };
@@ -301,6 +313,7 @@ async function fetchAdsbLolPointFallback(req) {
 async function serveAdsbLolPointFallback(req, res, requestedMode, reason) {
   const fallback = await fetchAdsbLolPointFallback(req);
   if (!fallback) return false;
+  const encoded = encodedBody(req, fallback.body, fallback.gzip);
   res.writeHead(200, {
     ...buildOpenSkyHeaders({
       cacheStatus: fallback.cacheStatus,
@@ -311,8 +324,9 @@ async function serveAdsbLolPointFallback(req, res, requestedMode, reason) {
     'X-Flight-Source': 'adsb.lol',
     'X-Flight-Coverage': `${ADSBLOL_POINT_RADIUS_NM}nm regional fallback`,
     'X-Flight-Count': String(fallback.count),
+    ...encoded.headers,
   });
-  res.end(fallback.body);
+  res.end(encoded.body);
   return true;
 }
 
@@ -380,9 +394,13 @@ export function openSkyProxy() {
             reason: 'cached',
           };
           const isStale = now - _openskyCacheTime >= _openskyTtlMs;
-          res.writeHead(
-            _openskyCacheStatus || 200,
-            buildOpenSkyHeaders({
+          const encoded = encodedBody(
+            req,
+            _openskyCacheBody,
+            _openskyCacheGzip,
+          );
+          res.writeHead(_openskyCacheStatus || 200, {
+            ...buildOpenSkyHeaders({
               cacheStatus: isStale ? 'STALE' : 'HIT',
               requestedMode: cachedMeta.requestedMode || requestedMode,
               usedMode: cachedMeta.usedMode || 'unknown',
@@ -396,8 +414,9 @@ export function openSkyProxy() {
                 ? (_openskyCooldownUntil - now) / 1000
                 : undefined,
             }),
-          );
-          res.end(_openskyCacheBody);
+            ...encoded.headers,
+          });
+          res.end(encoded.body);
           return;
         }
         // Cooling down with nothing cached (cold start into a rate limit):
@@ -507,7 +526,12 @@ export function openSkyProxy() {
         ) {
           // Keep the last global snapshot available as a fail-soft cache,
           // but do not label or render it as a fresh live result.
+          // The frame is built before either slot is written so the two are
+          // always published together: a request arriving mid-refresh reads the
+          // previous body with the previous body's frame, never a mismatch.
+          const frame = await gzipBody(body);
           _openskyCacheBody = body;
+          _openskyCacheGzip = frame;
           _openskyCacheStatus = upstream.status;
           _openskyCacheTime = now;
           _openskyCacheSourceEpochMs = sourceEpochMs;
@@ -532,9 +556,13 @@ export function openSkyProxy() {
           // Serve the last-good body instead of the 429 when we have one —
           // the layer keeps rendering (STALE-cued) instead of dying.
           if (_openskyCacheBody && _openskyCacheStatus === 200) {
-            res.writeHead(
-              200,
-              buildOpenSkyHeaders({
+            const encoded = encodedBody(
+              req,
+              _openskyCacheBody,
+              _openskyCacheGzip,
+            );
+            res.writeHead(200, {
+              ...buildOpenSkyHeaders({
                 cacheStatus: 'STALE',
                 requestedMode,
                 usedMode,
@@ -542,8 +570,9 @@ export function openSkyProxy() {
                 staleSeconds: (now - _openskyCacheTime) / 1000,
                 retryAfterSeconds: cooldownMs / 1000,
               }),
-            );
-            res.end(_openskyCacheBody);
+              ...encoded.headers,
+            });
+            res.end(encoded.body);
             return;
           }
         }
@@ -616,7 +645,10 @@ export function openSkyProxy() {
         // Only cache successful responses — error responses (401/403/429/5xx)
         // should not be served from cache on subsequent requests
         if (upstream.ok) {
+          // Published as a pair, for the reason given at the other cache fill.
+          const frame = await gzipBody(body);
           _openskyCacheBody = body;
+          _openskyCacheGzip = frame;
           _openskyCacheStatus = upstream.status;
           _openskyCacheTime = now;
           _openskyCacheSourceEpochMs = sourceEpochMs;
@@ -635,16 +667,26 @@ export function openSkyProxy() {
           _openskyCooldownUntil = 0;
         }
 
-        res.writeHead(
-          upstream.status,
-          buildOpenSkyHeaders({
+        // Only the branch above caches, so the gzip slot belongs to this body
+        // only when this body is the one it just stored. An error response
+        // (401/403/429/5xx) is not cached and must not borrow the previous
+        // snapshot's frame — identity comparison, not `upstream.ok`, because it
+        // stays correct if the caching condition above ever changes.
+        const encoded = encodedBody(
+          req,
+          body,
+          _openskyCacheBody === body ? _openskyCacheGzip : null,
+        );
+        res.writeHead(upstream.status, {
+          ...buildOpenSkyHeaders({
             cacheStatus: 'MISS',
             requestedMode,
             usedMode,
             reason,
           }),
-        );
-        res.end(body);
+          ...encoded.headers,
+        });
+        res.end(encoded.body);
       } catch (e) {
         console.error('[OpenSky Proxy]', e.message);
         if (_openskyCacheBody) {
@@ -655,17 +697,22 @@ export function openSkyProxy() {
             usedMode: 'unknown',
             reason: 'cached_stale',
           };
-          res.writeHead(
-            _openskyCacheStatus || 200,
-            buildOpenSkyHeaders({
+          const encoded = encodedBody(
+            req,
+            _openskyCacheBody,
+            _openskyCacheGzip,
+          );
+          res.writeHead(_openskyCacheStatus || 200, {
+            ...buildOpenSkyHeaders({
               cacheStatus: 'STALE',
               requestedMode:
                 cachedMeta.requestedMode || OPENSKY_AUTH_MODE_DEFAULT,
               usedMode: cachedMeta.usedMode || 'unknown',
               reason: cachedMeta.reason || 'cached_stale',
             }),
-          );
-          res.end(_openskyCacheBody);
+            ...encoded.headers,
+          });
+          res.end(encoded.body);
           return;
         }
         const requestedMode = normalizeOpenSkyAuthMode(