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_internal_test.go

@@ -11,11 +11,15 @@ import (
 // carries every hardening timeout wired onto it, including the slowloris
 // defense (ReadHeaderTimeout) and the keep-alive bound (IdleTimeout). This
 // guards against a field being defined but never set on the server, so
-// each assertion compares the server field to its constant.
+// each assertion compares the server field to its constant, or, for
+// WriteTimeout, to downstream_timeout from the config.
 func TestNewHTTPServerTimeouts(t *testing.T) {
 	t.Parallel()
 
-	s := &Server{config: &config.Config{Port: 8080}}
+	s := &Server{config: &config.Config{
+		Port:              8080,
+		DownstreamTimeout: 45 * time.Second,
+	}}
 
 	srv := s.newHTTPServer()
 
@@ -26,7 +30,7 @@ func TestNewHTTPServerTimeouts(t *testing.T) {
 	}{
 		{"ReadTimeout", srv.ReadTimeout, HTTPReadTimeout},
 		{"ReadHeaderTimeout", srv.ReadHeaderTimeout, HTTPReadHeaderTimeout},
-		{"WriteTimeout", srv.WriteTimeout, HTTPWriteTimeout},
+		{"WriteTimeout", srv.WriteTimeout, 45 * time.Second},
 		{"IdleTimeout", srv.IdleTimeout, HTTPIdleTimeout},
 	}