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/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();
+    }
 }