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/config_validation_internal_test.go

@@ -5,6 +5,7 @@ import (
 	"log/slog"
 	"os"
 	"path/filepath"
+	"slices"
 	"strings"
 	"testing"
 	"time"
@@ -216,6 +217,25 @@ func TestCommaSeparatedAllowlistStillSupported(t *testing.T) {
 	}
 }
 
+// TestAllowlistHostsAcceptsUnderscore checks that an upstream host name with
+// an underscore, which pixa can fetch from, is accepted as an entry.
+func TestAllowlistHostsAcceptsUnderscore(t *testing.T) {
+	t.Parallel()
+
+	c, err := configFromYAML(t, signingKeyLine+`allowlist_hosts:
+  - my_bucket.example.com
+  - .my_bucket.example.org
+`)
+	if err != nil {
+		t.Fatalf("host names with an underscore should load, got error: %v", err)
+	}
+
+	want := []string{"my_bucket.example.com", ".my_bucket.example.org"}
+	if !slices.Equal(c.AllowlistHosts, want) {
+		t.Errorf("AllowlistHosts = %v, want %v", c.AllowlistHosts, want)
+	}
+}
+
 // runAbortCases asserts that each case's config aborts startup with an
 // error message mentioning every expected substring.
 func runAbortCases(t *testing.T, cases []abortCase) {
@@ -318,6 +338,20 @@ func invalidHostAndCredentialCases() []abortCase {
 				keyAllowlistHosts, "example.com/images",
 			},
 		},
+		{
+			name: "allowlist host with wildcard",
+			yaml: signingKeyLine + "allowlist_hosts:\n  - \"*.example.com\"\n",
+			wantErrSubstrings: []string{
+				keyAllowlistHosts, "*.example.com",
+			},
+		},
+		{
+			name: "allowlist host with port",
+			yaml: signingKeyLine + "allowlist_hosts:\n  - example.com:8443\n",
+			wantErrSubstrings: []string{
+				keyAllowlistHosts, "example.com:8443",
+			},
+		},
 		{
 			name:              "allowlist host with whitespace",
 			yaml:              signingKeyLine + "allowlist_hosts:\n  - \"exa mple.com\"\n",