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

access_control_allow_origin, upstream_fetch_timeout,
upstream_max_response_size and downstream_timeout were in the README
but unknown to pixa, so a config that followed it 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 positive Go
duration strings; the size is a positive whole number of bytes; the
origin is * or one scheme and host. An invalid value aborts startup
naming the key and value. downstream_timeout replaces HTTPWriteTimeout
for the server's write timeout and the per-request timeout.

Model: opus-5-5

internal/handlers/handlers.go

@@ -106,6 +106,8 @@ func (s *Handlers) initImageService() error {
 	// Create the fetcher config
 	fetcherCfg := httpfetcher.DefaultConfig()
 	fetcherCfg.AllowHTTP = s.config.AllowHTTP
+	fetcherCfg.Timeout = s.config.UpstreamFetchTimeout
+	fetcherCfg.MaxResponseSize = s.config.UpstreamMaxResponseSize
 
 	if s.config.UpstreamConnectionsPerHost > 0 {
 		fetcherCfg.MaxConnectionsPerHost = s.config.UpstreamConnectionsPerHost