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/composables/useWaitlist.js

@@ -0,0 +1,31 @@
+import { ref } from 'vue'
+import api from '../axios.js'
+
+export function useWaitlist() {
+    const loading = ref(false)
+    const success = ref(false)
+    const error = ref(null)
+
+    async function submit(name, email, source = null) {
+        loading.value = true
+        error.value = null
+        try {
+            await api.post('/waitlist', {
+                name,
+                email,
+                source,
+                referrer: document.referrer || null,
+            })
+            success.value = true
+        } catch (e) {
+            const fieldErrors = e.response?.data?.errors
+            error.value = fieldErrors
+                ? Object.values(fieldErrors)[0][0]
+                : (e.response?.data?.message || 'Something went wrong — please try again.')
+        } finally {
+            loading.value = false
+        }
+    }
+
+    return { loading, success, error, submit }
+}