feat(subscription): email checkout reminders for expired sessions

Enable Stripe checkout recovery links and send rate-limited reminders to the
admin who started an unfinished subscription checkout.

database/migrations/2026_10_29_221743_add_checkout_reminder_sent_at_to_subscriptions_table.php

@@ -0,0 +1,22 @@
+<?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::table('subscriptions', function (Blueprint $table) {
+            $table->timestamp('stripe_checkout_reminder_sent_at')->nullable();
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('subscriptions', function (Blueprint $table) {
+            $table->dropColumn('stripe_checkout_reminder_sent_at');
+        });
+    }
+};