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(),
         ]);
     }
 }

app/Models/Site.php

@@ -10,6 +10,7 @@ class Site extends Model
     protected $fillable = [
         'collection',
         'domain',
+        'root_domain',
         'url',
         'title',
         'description',
@@ -35,4 +36,20 @@ class Site extends Model
     {
         return $this->hasMany(Backlink::class, 'target_domain', 'domain');
     }
+
+    /** Other indexed sites whose root domain is this site's domain, e.g. blog.example.com under example.com. */
+    public function subdomains(): HasMany
+    {
+        return $this->hasMany(Site::class, 'root_domain', 'domain')->where('id', '!=', $this->id);
+    }
+
+    /** The root-domain Site this one is a subdomain of, if it's indexed and if this site even is a subdomain. */
+    public function rootSite(): ?Site
+    {
+        if ($this->domain === $this->root_domain) {
+            return null;
+        }
+
+        return static::where('domain', $this->root_domain)->first();
+    }
 }

app/Services/CommonCrawl/CommonCrawlIndexService.php

@@ -6,6 +6,7 @@ use App\Models\Backlink;
 use App\Models\Site;
 use App\Models\SitePage;
 use App\Support\CommonCrawl\WarcRecordFetcher;
+use App\Support\RootDomain;
 use Illuminate\Support\Collection;
 use Illuminate\Support\Facades\Http;
 
@@ -116,6 +117,7 @@ class CommonCrawlIndexService
 
         $homepage = $pages->sort(fn ($a, $b) => $this->compareCaptures($a, $b))->first();
         $site->url = $homepage['url'];
+        $site->root_domain = RootDomain::of($group['domain']);
 
         if ($withContent) {
             $content = $this->fetchContent($homepage, $contentCache);

app/Support/RootDomain.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace App\Support;
+
+/**
+ * Naive "last two labels" root-domain heuristic — good enough for the common case
+ * (example.com, somesite.es) but wrong for real-world two-part public suffixes like
+ * .co.uk or .com.au, where it would treat "co.uk" as the root instead of
+ * "example.co.uk". Doing that correctly means integrating the Mozilla Public Suffix
+ * List; this is a deliberate simplification until that's worth the added complexity.
+ */
+class RootDomain
+{
+    public static function of(string $domain): string
+    {
+        $labels = explode('.', strtolower(trim($domain, '.')));
+
+        if (count($labels) <= 2) {
+            return implode('.', $labels);
+        }
+
+        return implode('.', array_slice($labels, -2));
+    }
+}