Withhold a site name that could be the recipe link's host

A page that names no site is stored under its host, which may be the
pasted link's (a token in a subdomain) or a redirect's. The dinner
detail now returns source.site_name only when the stored name could not
be a host: the link's own host, an IP address or a dotted host name is
withheld as null, so "Recipe from {site}" shows only real attribution.
The extractor's host fallback is shared as recipes.HostSiteName.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/server/dinners.go

@@ -72,7 +72,11 @@ func getDinners(logger *slog.Logger, svc *dinners.Service) http.HandlerFunc {
 
 // getDinner answers GET /dinners/{id} with the dinner, its recipe and the
 // reason it was suggested. An archived dinner is 404, like an unknown one,
-// not a 200 with its status: it has no detail to show.
+// not a 200 with its status: it has no detail to show. source never has the
+// recipe page's link, and source.site_name is null unless the page itself
+// named its site — a name that could be a host, the link's or a redirect's,
+// is withheld (dinners.Source) — so the detail screen shows "Recipe from
+// {site}" only when site_name is not null.
 func getDinner(logger *slog.Logger, svc *dinners.Service) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		d, err := svc.Get(r.Context(), chi.URLParam(r, "id"))