Record site-name provenance instead of inferring it from the name's shape

Recipe extraction no longer falls back to the URL's host: a stored
recipes.site_name is now always a name the page gave (publisher or
og:site_name), and null when it named none. dinners.SourceOf is the one
documented rule for every fetched recipe's source: the page's name is
shown as it stands; otherwise the site is named by the link's host only
when the link itself is shown as free of secrets, else null. The recipe
detail, idea results and the recipe-candidate offer endpoint all apply
it; candidates no longer carry an extraction-time hostname fallback, so
their site_name is nullable (frontend updated to match).

Migration 0020 clears host-shaped site names (dotted host names, IP
addresses) stored before the change, in recipes and in legacy idea job
results, so already-stored rows read under the same rule.

Maestro 966066e3-0e25-4b8b-9bb0-0694c47c9106

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

internal/dinners/grounding_test.go

@@ -718,11 +718,11 @@ func TestCandidates(t *testing.T) {
 	}
 	first, second := got[0], got[1]
 	wantPhoto := dinners.PhotoURL(e.dinner(t, c.ID).PhotoID.String)
-	if first.Title != "Chicken Curry" || first.SiteName != "Curry House" ||
+	if first.Title != "Chicken Curry" || deref(first.SiteName) != "Curry House" ||
 		first.PhotoURL == nil || *first.PhotoURL != wantPhoto || first.Minutes == nil || *first.Minutes != 40 {
 		t.Errorf("first = %+v, want the stored curryA with photo %s", first, wantPhoto)
 	}
-	if second.Title != "Easy Weeknight Curry" || second.SiteName != "B Kitchen" ||
+	if second.Title != "Easy Weeknight Curry" || deref(second.SiteName) != "B Kitchen" ||
 		second.PhotoURL != nil || second.Minutes == nil || *second.Minutes != 25 {
 		t.Errorf("second = %+v, want curryB without a photo", second)
 	}