Track and display subdomains under their root domain

Adds sites.root_domain (indexed string, backfilled for existing rows)
computed via a naive "last two labels" heuristic (App\Support\RootDomain)
— good enough for example.com/somesite.es, but wrong for two-part
public suffixes like .co.uk. Doing that correctly needs the Mozilla
Public Suffix List; flagged as a known simplification rather than
built now.

Deliberately not a hard FK: like Backlink.target_domain, a subdomain
can get indexed before its root domain does (or vice versa), so
Site::subdomains()/rootSite() match on the domain string rather than
requiring the relationship to exist at write time.

Site detail page: a root domain shows a paginated "Subdomains"
section (same guest-preview cap as Pages/Backlinks); a subdomain
shows "Subdomain of {root}" linking back, when the root is itself
indexed.

Verified against real data: 21k.ro correctly lists its two known
subdomains (buzau.21k.ro, timisoara.21k.ro), each of which correctly
links back to 21k.ro. Added SubdomainsTest (root lists subdomains,
subdomain links to root, empty state) — full suite (31 tests) passes.

app/Http/Controllers/SiteController.php

@@ -21,12 +21,20 @@ class SiteController extends Controller
         $referringDomainsCount = $site->inboundLinks()->distinct()->count('site_id');
         $outboundLinksCount = $site->outboundLinks()->count();
 
+        $subdomains = GuestPreview::paginate(
+            $site->subdomains()->withCount('pages')->orderBy('domain'),
+            50,
+            'subdomains_page'
+        );
+
         return view('sites.show', [
             'site' => $site,
             'pages' => $pages,
             'backlinks' => $backlinks,
             'referringDomainsCount' => $referringDomainsCount,
             'outboundLinksCount' => $outboundLinksCount,
+            'subdomains' => $subdomains,
+            'rootSite' => $site->rootSite(),
         ]);
     }
 }