A handler's panic is logged and counted as 500 (closes #125)

Recoverer comes first, so it wrote the 500 only after the panic had
passed the request log and the metrics, which both recorded 200.
Write500OnPanic, placed after the request log and after the metrics,
writes the 500 as the panic passes and panics again, so Sentry and
Recoverer still get it. Recoverer now writes its 500 only when no
status was written, so the server logs no superfluous WriteHeader call.
The test also requires Recoverer's log line for the handler's panic, so
it fails if Write500OnPanic stops passing the panic on.

Model: opus-5-5

TODO.md

@@ -21,6 +21,12 @@ Run `make frontend-viewport-test` in CI as its own step; it is not part of
 
 # Completed Steps
 
+- 2026-10-14: a request whose handler panics is logged with status 500 and
+  counted with `code` 500 in the metrics
+  ([#125](https://git.eeqj.de/sneak/netwatch/issues/125)), not 200. `Recoverer`
+  is still the first middleware; `Write500OnPanic`, after the request log and
+  after the metrics, writes the 500 as the panic passes and lets it go on to
+  Sentry and `Recoverer`, which now writes a 500 only when no status was written
 - 2026-10-14: with `TRUSTED_PROXIES` unset, the container's nginx trusts the
   three private ranges of RFC 1918, `10.0.0.0/8`, `172.16.0.0/12` and
   `192.168.0.0/16` ([#119](https://git.eeqj.de/sneak/netwatch/issues/119)), as