a gzipped response was handed on still labelled gzipped

`fetch` decompresses a response body before anyone sees it, and the headers it
returns still say `content-encoding: gzip`. Copying those onto the decoded bytes
tells the client to gunzip plain JSON: `Decompression error: ZlibError`, which
surfaced as a 502 from the catch that wraps the forward.

It hides well, and that is the part worth writing down. A 401 body is a few bytes
and is not compressed, so every unauthenticated probe passes. The first request
carrying a real credential is the first one big enough to be gzipped, and the
first one to fail - which reads as "the credential broke it".

Measured on alpha in exactly that order: the extension door answered 401 without
a token and 502 with one.

`content-length` goes too. It describes the compressed length and the body being
handed on is not that length.

Response headers are now a different function from request headers, because they
are a different question and treating them as one is what caused this.

sidecar/index.ts

@@ -1,6 +1,6 @@
 import { createSidecarConnector } from '@@/sidecar/connect';
 import { API_URL } from '@@/officer-url.mjs';
-import { getLinkwardenBase, stripHopByHop, redactPath } from './upstream';
+import { getLinkwardenBase, stripHopByHop, stripResponseHeaders, redactPath } from './upstream';
 
 // The officer-linkwarden sidecar: a singleton process that registers with the API server and owns a
 // transparent reverse-proxy to a self-hosted Linkwarden. The platform API is a thin forwarder to this;
@@ -86,7 +86,7 @@ Bun.serve({
 
     return new Response(upstream.body, {
       status: upstream.status,
-      headers: stripHopByHop(upstream.headers),
+      headers: stripResponseHeaders(upstream.headers),
     });
   },
 });

sidecar/upstream.ts

@@ -79,6 +79,29 @@ export function stripHopByHop(src: Headers): Headers {
   return out;
 }
 
+/**
+ * Headers for a response we are handing back, which is NOT the same as headers for a request.
+ *
+ * ── `content-encoding` has to go, and finding out why cost a 502 ──
+ *
+ * `fetch` decompresses a gzipped response body before anyone sees it. The headers it returns still say
+ * `content-encoding: gzip`, so copying them onto the decoded bytes tells the client to gunzip plain
+ * JSON — `Decompression error: ZlibError`, surfacing as a 502 from the catch below.
+ *
+ * It hides well. A 401 body is a few bytes and is not compressed, so every unauthenticated probe passes
+ * and the first REAL request is the one that fails. Measured exactly that way on alpha: the door
+ * answered 401 correctly without a token and 502 with one.
+ *
+ * `content-length` goes with it for the same reason — it describes the compressed length, and the body
+ * being handed on is not that length.
+ */
+export function stripResponseHeaders(src: Headers): Headers {
+  const out = stripHopByHop(src);
+  out.delete('content-encoding');
+  out.delete('content-length');
+  return out;
+}
+
 /*
   Paths are logged; tokens are not.