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.

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),