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/Http/Requests/Api/StoreWaitlistRequest.php

@@ -0,0 +1,26 @@
+<?php
+
+namespace App\Http\Requests\Api;
+
+use Illuminate\Foundation\Http\FormRequest;
+
+class StoreWaitlistRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    /**
+     * @return array<string, array<int, string>>
+     */
+    public function rules(): array
+    {
+        return [
+            'name' => ['required', 'string', 'max:255'],
+            'email' => ['required', 'email', 'max:255'],
+            'source' => ['nullable', 'string', 'max:64'],
+            'referrer' => ['nullable', 'string', 'max:2048'],
+        ];
+    }
+}