Send only the Referer's scheme and host to Sentry (closes #548)

A Sentry event kept the Referer header verbatim, and a Referer can be
an entrypoint URL: a sender that follows the README's http:// to
https:// redirect sends the URL it was redirected from, and a browser
sends the 404 page's URL under /h/ when it fetches that page's
stylesheet. The Sentry hook now keeps only the Referer's scheme and
host. The access log's reduction moves, unchanged, into a new package,
internal/referer, as SchemeAndHost; the access log and the Sentry hook
both call it there. The comment above sentryKeepsHeader and the
README's Sentry passage say so, and the README's Package Layout lists
the new package.

Model: opus-5-5

internal/server/sentry.go

@@ -8,6 +8,7 @@ import (
 
 	"github.com/getsentry/sentry-go"
 	"github.com/go-chi/chi"
+	"sneak.berlin/go/webhooker/internal/referer"
 )
 
 // sentryRedacted stands in for a withheld field on every event shipped
@@ -253,6 +254,7 @@ func sentryTransactionName(name, pattern string) string {
 // gorilla/csrf accepts in place of the form field — and the shared
 // secrets senders put on the receiver route (X-Gitlab-Token and the
 // per-provider signature headers) would otherwise ship verbatim.
+// The Referer it keeps is cut down to its scheme and host.
 func keptSentryHeaders(headers map[string]string) map[string]string {
 	if len(headers) == 0 {
 		return headers
@@ -261,9 +263,15 @@ func keptSentryHeaders(headers map[string]string) map[string]string {
 	kept := make(map[string]string, len(headers))
 
 	for name, value := range headers {
-		if sentryKeepsHeader(name) {
-			kept[name] = value
+		if !sentryKeepsHeader(name) {
+			continue
 		}
+
+		if http.CanonicalHeaderKey(name) == "Referer" {
+			value = referer.SchemeAndHost(value)
+		}
+
+		kept[name] = value
 	}
 
 	return kept
@@ -271,11 +279,14 @@ func keptSentryHeaders(headers map[string]string) map[string]string {
 
 // sentryKeepsHeader reports whether a request header is routing or
 // content metadata rather than client-chosen payload. Referer is kept
-// on the reasoning that it is browser-set, that the only query
-// parameters in this service's own URLs are the login page's `next`,
-// `notice` and the event log's `show`, and that Referrer-Policy is set to
-// strict-origin-when-cross-origin. X-Request-Id ties the event to the
-// local access log line, which holds the rest of the detail.
+// only as its scheme and host, through referer.SchemeAndHost, which
+// the access log uses too: its path can be an entrypoint URL, which
+// carries the UUID, as when a sender follows a redirect from http://
+// to https:// or a browser fetches the 404 page's stylesheet from
+// under /h/. The scheme and host are all gorilla/csrf compares a
+// Referer by, so a CSRF rejection is still readable from the event.
+// X-Request-Id ties the event to the local access log line, which
+// holds the rest of the detail.
 func sentryKeepsHeader(name string) bool {
 	switch http.CanonicalHeaderKey(name) {
 	case "Accept",