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/Services/WaitlistService.php

@@ -0,0 +1,29 @@
+<?php
+
+namespace App\Services;
+
+use App\Models\WaitlistSubscriber;
+
+final class WaitlistService
+{
+    /**
+     * Add someone to the feature-launch waitlist.
+     *
+     * Idempotent: re-subscribing an existing email is a no-op that returns the
+     * original subscriber unchanged — original meta (source, referrer) is kept,
+     * never a duplicate row or an error.
+     */
+    public function subscribe(
+        string $name,
+        string $email,
+        ?string $source = null,
+        ?string $referrer = null,
+    ): WaitlistSubscriber {
+        $email = strtolower(trim($email));
+
+        return WaitlistSubscriber::firstOrCreate(
+            ['email' => $email],
+            ['name' => $name, 'source' => $source, 'referrer' => $referrer],
+        );
+    }
+}