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>

resources/js/components/PricingGrid.vue

@@ -80,13 +80,16 @@
                     <button v-else type="button" disabled class="w-full py-3 px-4 bg-zinc-100 rounded-xl text-center font-bold text-zinc-400 cursor-not-allowed">Coming soon</button>
                 </div>
             </div>
+
+            <WaitlistForm v-if="hasComingSoon" source="pricing" />
         </div>
     </section>
 </template>
 
 <script setup>
-import { ref } from 'vue'
+import { computed, ref } from 'vue'
 import { useAuth } from '../composables/useAuth.js'
+import WaitlistForm from './WaitlistForm.vue'
 
 const { isAuthenticated, userTier } = useAuth()
 
@@ -102,6 +105,8 @@ const PRICE_SUFFIX = { monthly: '/mo', annual: '/yr' }
 // disabled until then. Remove a tier from this list to make its button live.
 const COMING_SOON = ['basic', 'plus']
 
+const hasComingSoon = computed(() => COMING_SOON.length > 0)
+
 function isComingSoon(tier) {
     return COMING_SOON.includes(tier)
 }