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>

docs/ARCHITECTURE.md

@@ -215,9 +215,15 @@ automatic** (D-021).
   `pending` or `failed`; a recipe entered by hand has only `raw_text`.
   Two items at one position leave recipe order unknown, so such a recipe
   is 500 rather than shown out of order. `source` is `{site_name}` of the
-  page the recipe was fetched from (`site_name` null when the page names
-  none), null for a manual recipe or none; it has no `url`, as the page's
-  link may carry a personal token. `why_picked` is the reason of the
+  page the recipe was fetched from, null for a manual recipe or none; it
+  has no `url`, as the page's link may carry a personal token.
+  `site_name` is the name the page gives its site (publisher or
+  `og:site_name`) and null otherwise: a page naming no site is stored
+  under its host, which may be the pasted link's — a token in a
+  subdomain — or a redirect's, so any stored name that could be a host
+  (the link's host, an IP address or a dotted host name, even a real
+  name like "Food.com") is withheld. The detail screen shows "Recipe
+  from {site}" only when `site_name` is not null. `why_picked` is the reason of the
   suggestion that created the dinner — its pending or accepted one, as
   declining a suggestion archives its dinner — null for one no suggestion
   created; a dinner with more than one such suggestion is 500, as nothing