Refuse image requests whose Referer is on referer_blocklist (closes #90)

A new setting, referer_blocklist (PIXA_REFERER_BLOCKLIST), lists hosts
whose pages may not show pixa's images. Entries are written and matched
as allowlist_hosts are, with the same matcher. Both image routes check
the Referer before the signature, the cache and the upstream fetch, and
answer 403 with the JSON error, so a blocked request costs nothing and
is refused whether or not the image is cached. No Referer, or one that
does not parse, is served; README.md says this makes the list easy to
get around. An entry of either host list that is not a host name
(letters, digits, hyphens, underscores, dots, at most one leading dot)
or an IP address now aborts startup naming the setting and the entry.

Model: opus-5-5

internal/config/env_internal_test.go

@@ -65,6 +65,7 @@ func TestEnvironmentSetsEveryKey(t *testing.T) {
 	t.Setenv("PIXA_METRICS_PASSWORD", "metricspass")
 	t.Setenv("PIXA_SIGNING_KEY", validTestSigningKey)
 	t.Setenv("PIXA_ALLOWLIST_HOSTS", "s3.sneak.cloud,.example.com")
+	t.Setenv("PIXA_REFERER_BLOCKLIST", "hotlinker.example,.leech.example")
 	t.Setenv("PIXA_ALLOW_HTTP", "true")
 	t.Setenv("PIXA_UPSTREAM_CONNECTIONS_PER_HOST", "5")
 	t.Setenv("PIXA_UPSTREAM_CONNECTIONS", "10")
@@ -93,6 +94,7 @@ func TestEnvironmentSetsEveryKey(t *testing.T) {
 		MetricsPassword:            "metricspass",
 		SigningKey:                 validTestSigningKey,
 		AllowlistHosts:             []string{testHostS3, ".example.com"},
+		RefererBlocklist:           []string{"hotlinker.example", ".leech.example"},
 		AllowHTTP:                  true,
 		UpstreamConnectionsPerHost: 5,
 		UpstreamConnections:        10,