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>

database/factories/WaitlistSubscriberFactory.php

@@ -0,0 +1,25 @@
+<?php
+
+namespace Database\Factories;
+
+use App\Models\WaitlistSubscriber;
+use Illuminate\Database\Eloquent\Factories\Factory;
+
+/**
+ * @extends Factory<WaitlistSubscriber>
+ */
+class WaitlistSubscriberFactory extends Factory
+{
+    /**
+     * Define the model's default state.
+     *
+     * @return array<string, mixed>
+     */
+    public function definition(): array
+    {
+        return [
+            'name' => fake()->name(),
+            'email' => fake()->unique()->safeEmail(),
+        ];
+    }
+}