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/Http/Controllers/Api/WaitlistController.php

@@ -0,0 +1,27 @@
+<?php
+
+namespace App\Http\Controllers\Api;
+
+use App\Http\Controllers\Controller;
+use App\Http\Requests\Api\StoreWaitlistRequest;
+use App\Services\WaitlistService;
+use Illuminate\Http\JsonResponse;
+
+class WaitlistController extends Controller
+{
+    public function __construct(private readonly WaitlistService $waitlist) {}
+
+    public function store(StoreWaitlistRequest $request): JsonResponse
+    {
+        $this->waitlist->subscribe(
+            name: $request->string('name')->toString(),
+            email: $request->string('email')->toString(),
+            source: $request->filled('source') ? $request->string('source')->toString() : null,
+            referrer: $request->filled('referrer') ? $request->string('referrer')->toString() : null,
+        );
+
+        return response()->json([
+            'message' => "You're on the list — we'll email you when alerts go live.",
+        ], 201);
+    }
+}