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>

app/Filament/Resources/WaitlistSubscriberResource/Pages/ListWaitlistSubscribers.php

@@ -0,0 +1,42 @@
+<?php
+
+namespace App\Filament\Resources\WaitlistSubscriberResource\Pages;
+
+use App\Filament\Resources\WaitlistSubscriberResource;
+use App\Models\WaitlistSubscriber;
+use Filament\Actions\Action;
+use Filament\Resources\Pages\ListRecords;
+use Symfony\Component\HttpFoundation\StreamedResponse;
+
+class ListWaitlistSubscribers extends ListRecords
+{
+    protected static string $resource = WaitlistSubscriberResource::class;
+
+    protected function getHeaderActions(): array
+    {
+        return [
+            Action::make('export')
+                ->label('Export CSV')
+                ->icon('heroicon-o-arrow-down-tray')
+                ->action(fn (): StreamedResponse => response()->streamDownload(function (): void {
+                    $handle = fopen('php://output', 'wb');
+
+                    fputcsv($handle, ['name', 'email', 'source', 'referrer', 'joined_at']);
+
+                    WaitlistSubscriber::query()
+                        ->orderBy('created_at')
+                        ->each(function (WaitlistSubscriber $subscriber) use ($handle): void {
+                            fputcsv($handle, [
+                                $subscriber->name,
+                                $subscriber->email,
+                                $subscriber->source,
+                                $subscriber->referrer,
+                                $subscriber->created_at?->toDateTimeString(),
+                            ]);
+                        });
+
+                    fclose($handle);
+                }, 'waitlist.csv', ['Content-Type' => 'text/csv'])),
+        ];
+    }
+}