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>

database/migrations/2026_06_12_085857_create_waitlist_subscribers_table.php

@@ -0,0 +1,31 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::create('waitlist_subscribers', function (Blueprint $table) {
+            $table->id();
+            $table->string('name');
+            $table->string('email')->unique();
+            $table->string('source', 64)->nullable()->comment('Where they joined from, e.g. pricing');
+            $table->text('referrer')->nullable()->comment('document.referrer at signup');
+            $table->timestamps();
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::dropIfExists('waitlist_subscribers');
+    }
+};