Add opt-in custom domains with automatic SSL

Customers can connect their own domain to a events page (storefront/event):
add a domain, point an A record (apex + www) to the app server, click Verify —
DNS is checked, then Ladill Domains' central SSL service issues + installs the
Let's Encrypt cert and calls back to flip it live. The custom domain then serves
the mapped page (host resolution on /). Feature-gated: only active when a Domains
SSL API key is set, so this deploy is inert until wired.

- custom_domains table + CustomDomain model
- CustomDomainService (DNS verify, request cert), DomainsSslClient, DnsResolver
- settings UI panel, signed SSL callback receiver, host resolution on /
- feature tests (DNS verify/fail, signed callback, ownership)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/Http/Controllers/Api/SslCallbackController.php

@@ -0,0 +1,43 @@
+<?php
+
+namespace App\Http\Controllers\Api;
+
+use App\Http\Controllers\Controller;
+use App\Services\CustomDomain\CustomDomainService;
+use Illuminate\Http\JsonResponse;
+use Illuminate\Http\Request;
+
+/**
+ * Signed completion callback from Ladill Domains' SSL service. Flips a custom
+ * domain to live (or failed) once the certificate is issued. Auth is the HMAC
+ * signature over the raw body (shared SSL_CALLBACK_SECRET), not a session.
+ */
+class SslCallbackController extends Controller
+{
+    public function __invoke(Request $request, CustomDomainService $service): JsonResponse
+    {
+        $secret = (string) config('customdomain.callback_secret');
+        $body = $request->getContent();
+        $signature = (string) $request->header('X-Ladill-Signature', '');
+
+        if ($secret === '' || ! hash_equals(hash_hmac('sha256', $body, $secret), $signature)) {
+            return response()->json(['error' => 'Invalid signature.'], 401);
+        }
+
+        $payload = json_decode($body, true);
+        $data = (array) ($payload['data'] ?? []);
+        $host = (string) ($data['host'] ?? '');
+        if ($host === '') {
+            return response()->json(['error' => 'Missing host.'], 422);
+        }
+
+        $service->applyCallback(
+            $host,
+            (string) ($data['status'] ?? 'failed'),
+            $data['expires_at'] ?? null,
+            $data['last_error'] ?? null,
+        );
+
+        return response()->json(['status' => 'ok']);
+    }
+}