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_test.go

@@ -325,6 +325,37 @@ func TestSentryScrub_ReplacesTheCapabilityPathWithTheRoutePattern(
 	)
 }
 
+// TestSentryScrub_KeepsOnlyTheRefererSchemeAndHost sends a receiver
+// request whose Referer is the entrypoint URL, as a sender's is after
+// it follows the README's redirect from http:// to https://. The event
+// keeps the Referer's scheme and host, and carries the UUID nowhere.
+func TestSentryScrub_KeepsOnlyTheRefererSchemeAndHost(t *testing.T) {
+	t.Parallel()
+
+	event := onlyEvent(t, sentryCase{
+		scrub:  true,
+		panics: true,
+		request: func(client *sentry.Client) *http.Request {
+			req := sentryReceiverRequest(client)
+			req.Header.Set(
+				"Referer",
+				"http://webhooker.example/h/"+sentryReceiverUUID,
+			)
+
+			return req
+		},
+	}.capture(t))
+
+	assert.NotContains(
+		t, marshalEvent(t, event), sentryReceiverUUID,
+	)
+	assert.Equal(
+		t,
+		"http://webhooker.example",
+		event.Request.Headers["Referer"],
+	)
+}
+
 // TestSentryScrub_KeepsTheRoutingContext checks the hook does not cost
 // the debugging signal: the route, its scheme and host, the method and
 // the metadata headers still identify what failed. On a static route