Add pricing-page waitlist (name + email signup)

Replaces the disabled "Coming soon" buttons on the pricing page with a
waitlist band so visitors can be notified when alerts launch — separate
from registered users.

- waitlist_subscribers table (name, email unique, source, referrer)
- WaitlistService::subscribe — normalises email, idempotent
- Public POST /api/waitlist (throttle:10,1), thin controller + form request
- Read-only Filament resource with streamed CSV export
- Vue: useWaitlist composable + WaitlistForm, rendered below the grid
  while any tier is still "coming soon"; sends source + document.referrer

Announcement send mechanism deferred to a later task.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/Feature/Admin/WaitlistSubscriberResourceTest.php

@@ -0,0 +1,31 @@
+<?php
+
+use App\Filament\Resources\WaitlistSubscriberResource\Pages\ListWaitlistSubscribers;
+use App\Models\User;
+use App\Models\WaitlistSubscriber;
+use Livewire\Livewire;
+
+beforeEach(function () {
+    $this->actingAs(User::factory()->admin()->create());
+});
+
+it('lists waitlist subscribers', function () {
+    $subscribers = WaitlistSubscriber::factory()->count(3)->create();
+
+    Livewire::test(ListWaitlistSubscribers::class)
+        ->assertOk()
+        ->assertCanSeeTableRecords($subscribers);
+});
+
+it('exposes a CSV export header action', function () {
+    Livewire::test(ListWaitlistSubscribers::class)
+        ->assertActionExists('export');
+});
+
+it('exports a CSV download when the action runs against real subscribers', function () {
+    WaitlistSubscriber::factory()->count(2)->create();
+
+    Livewire::test(ListWaitlistSubscribers::class)
+        ->callAction('export')
+        ->assertFileDownloaded('waitlist.csv');
+});