Add the four settings the README documented but pixa lacked (closes #61)

README.md documented access_control_allow_origin,
upstream_fetch_timeout, upstream_max_response_size and
downstream_timeout, but pixa did not know them, so a config following
the README aborted startup. Each is now a setting with its PIXA_
variable, defaulting to the value that was fixed in the code: *, 30s,
50 MiB and 60s. Durations are Go duration strings and must be positive;
the size is whole bytes, at most 1 GiB. The origin is * or one http or
https origin written exactly as a browser sends it; anything else
aborts startup. downstream_timeout sets both the server's write timeout
and the per-request timeout. The owner approved the edits to existing
tests.

Model: opus-5-5

internal/server/http.go

@@ -14,7 +14,6 @@ const (
 	// short, so a slowloris client dribbling headers is dropped well
 	// before it ties up a connection for the whole ReadTimeout window.
 	HTTPReadHeaderTimeout = 10 * time.Second
-	HTTPWriteTimeout      = 60 * time.Second
 	// HTTPIdleTimeout bounds how long an idle keep-alive connection is
 	// held open, so idle connections cannot accumulate without limit on a
 	// service targeting high concurrency.
@@ -30,7 +29,7 @@ func (s *Server) newHTTPServer() *http.Server {
 		Addr:              fmt.Sprintf(":%d", s.config.Port),
 		ReadTimeout:       HTTPReadTimeout,
 		ReadHeaderTimeout: HTTPReadHeaderTimeout,
-		WriteTimeout:      HTTPWriteTimeout,
+		WriteTimeout:      s.config.DownstreamTimeout,
 		IdleTimeout:       HTTPIdleTimeout,
 		MaxHeaderBytes:    HTTPMaxHeaderBytes,
 		Handler:           s,