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/Models/WaitlistSubscriber.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace App\Models;
+
+use Database\Factories\WaitlistSubscriberFactory;
+use Illuminate\Database\Eloquent\Attributes\Fillable;
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+
+#[Fillable(['name', 'email', 'source', 'referrer'])]
+class WaitlistSubscriber extends Model
+{
+    /** @use HasFactory<WaitlistSubscriberFactory> */
+    use HasFactory;
+}