Add opt-in custom domains with automatic SSL

Customers can connect their own domain to a merchant 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>

database/migrations/2026_06_26_210000_create_custom_domains_table.php

@@ -0,0 +1,33 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::create('custom_domains', function (Blueprint $table) {
+            $table->id();
+            $table->foreignId('qr_code_id')->constrained('qr_codes')->cascadeOnDelete(); // the storefront
+            $table->foreignId('user_id')->constrained()->cascadeOnDelete();
+            $table->string('host')->unique();
+            $table->boolean('include_www')->default(true);
+            $table->string('status', 16)->default('pending');     // pending | active | failed
+            $table->string('ssl_status', 16)->default('pending');  // pending | active | failed
+            $table->timestamp('dns_verified_at')->nullable();
+            $table->timestamp('ssl_issued_at')->nullable();
+            $table->timestamp('ssl_expires_at')->nullable();
+            $table->text('last_error')->nullable();
+            $table->timestamps();
+
+            $table->index('qr_code_id');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::dropIfExists('custom_domains');
+    }
+};