Drop Set-Cookie from the recovered 500 (closes #193)

When a handler sets a cookie and then panics before sending
anything, the recover middleware now deletes Set-Cookie before
writing its 500, so a request that failed never hands the client
a credential. Every other header, Location included, is left as
http.Error leaves it, matching chi's Recoverer. A response that
was already sent is untouched.

Tests cover the uncommitted case (no cookie, Location kept) and
assert the cookie still reaches the client when the response was
committed before the panic.

Model: opus-5-5

internal/middleware/recoverer.go

@@ -133,6 +133,11 @@ func (w *recoverResponseWriter) Unwrap() http.ResponseWriter {
 // what the access log records and the metrics count, and outside the
 // sentryhttp handler, whose Repanic option depends on something
 // further out recovering what it re-raises.
+//
+// Unlike http.Error on its own, it deletes any Set-Cookie the handler
+// set before panicking, because a request that failed must not hand
+// the client a credential; every other header is left to http.Error.
+// See https://git.eeqj.de/sneak/webhooker/issues/193.
 func (s *Middleware) Recoverer() func(http.Handler) http.Handler {
 	return func(next http.Handler) http.Handler {
 		return http.HandlerFunc(func(
@@ -164,6 +169,8 @@ func (s *Middleware) Recoverer() func(http.Handler) http.Handler {
 					return
 				}
 
+				rw.Header().Del("Set-Cookie")
+
 				http.Error(
 					rw,
 					http.StatusText(