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>

tests/Feature/Api/WaitlistTest.php

@@ -0,0 +1,64 @@
+<?php
+
+use App\Models\WaitlistSubscriber;
+
+it('stores a subscriber and returns 201', function () {
+    $response = $this->postJson('/api/waitlist', [
+        'name' => 'Ada Lovelace',
+        'email' => 'ada@example.com',
+    ]);
+
+    $response->assertCreated()
+        ->assertJsonStructure(['message']);
+
+    $this->assertDatabaseHas('waitlist_subscribers', [
+        'name' => 'Ada Lovelace',
+        'email' => 'ada@example.com',
+    ]);
+});
+
+it('treats a duplicate email as success without creating a second row', function () {
+    WaitlistSubscriber::factory()->create(['email' => 'ada@example.com']);
+
+    $this->postJson('/api/waitlist', [
+        'name' => 'Someone Else',
+        'email' => 'ada@example.com',
+    ])->assertCreated();
+
+    expect(WaitlistSubscriber::where('email', 'ada@example.com')->count())->toBe(1);
+});
+
+it('stores the source and referrer sent with the request', function () {
+    $this->postJson('/api/waitlist', [
+        'name' => 'Ada Lovelace',
+        'email' => 'ada@example.com',
+        'source' => 'pricing',
+        'referrer' => 'https://duckduckgo.com/',
+    ])->assertCreated();
+
+    $this->assertDatabaseHas('waitlist_subscribers', [
+        'email' => 'ada@example.com',
+        'source' => 'pricing',
+        'referrer' => 'https://duckduckgo.com/',
+    ]);
+});
+
+it('requires a name', function () {
+    $this->postJson('/api/waitlist', [
+        'email' => 'ada@example.com',
+    ])->assertStatus(422)->assertJsonValidationErrors('name');
+});
+
+it('rejects an invalid email', function () {
+    $this->postJson('/api/waitlist', [
+        'name' => 'Ada Lovelace',
+        'email' => 'not-an-email',
+    ])->assertStatus(422)->assertJsonValidationErrors('email');
+});
+
+it('throttles the endpoint', function () {
+    $route = collect(app('router')->getRoutes())
+        ->first(fn ($route) => $route->uri() === 'api/waitlist' && in_array('POST', $route->methods(), true));
+
+    expect($route?->gatherMiddleware())->toContain('throttle:10,1');
+});