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/middleware/middleware.go

@@ -22,6 +22,7 @@ import (
 	"sneak.berlin/go/webhooker/internal/config"
 	"sneak.berlin/go/webhooker/internal/globals"
 	"sneak.berlin/go/webhooker/internal/logfield"
+	"sneak.berlin/go/webhooker/internal/referer"
 	"sneak.berlin/go/webhooker/internal/session"
 )
 
@@ -48,10 +49,6 @@ const (
 	// pick the size of the line it writes.
 	redactedQuery = "?(redacted)"
 
-	// redactedReferer stands in for a Referer the access log cannot
-	// reduce to its scheme and host; see accessLogReferer.
-	redactedReferer = "(redacted)"
-
 	// maxLogRequestIDBytes bounds the request id, which is also
 	// client-supplied: chi's RequestID middleware passes an inbound
 	// X-Request-Id header through verbatim. Its generated form is an
@@ -342,27 +339,6 @@ func accessLogURL(r *http.Request, status int) string {
 	return pattern
 }
 
-// accessLogReferer returns the value for the access log's referer
-// field: the Referer's scheme and host, never its path or query. The
-// path can be an entrypoint URL, which carries the UUID: a sender that
-// follows a redirect from http:// to https:// 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. A Referer without both a
-// scheme and a host is logged as redactedReferer.
-func accessLogReferer(r *http.Request) string {
-	referer := r.Referer()
-	if referer == "" {
-		return ""
-	}
-
-	parsed, err := url.Parse(referer)
-	if err != nil || parsed.Scheme == "" || parsed.Host == "" {
-		return redactedReferer
-	}
-
-	return parsed.Scheme + "://" + parsed.Host
-}
-
 // Logging returns middleware that logs each HTTP request with
 // timing and metadata.
 func (s *Middleware) Logging() func(http.Handler) http.Handler {
@@ -413,7 +389,8 @@ func (s *Middleware) Logging() func(http.Handler) http.Handler {
 						requestID, maxLogRequestIDBytes,
 					),
 					"referer", logfield.Truncate(
-						accessLogReferer(r), logfield.MaxBytes,
+						referer.SchemeAndHost(r.Referer()),
+						logfield.MaxBytes,
 					),
 					"proto", r.Proto,
 					"remoteIP", RemoteIP(r),