Fix whatsapp_messages migration: drop FK constraints to missing tables

The demo_academy DB doesn't have organizations/users tables with those
exact names. Use plain indexed columns instead.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

database/migrations/2024_01_01_000078_create_whatsapp_messages_table.php

@@ -11,7 +11,7 @@ public function up(): void
     {
         Schema::create('whatsapp_messages', function (Blueprint $table) {
             $table->id();
-            $table->foreignId('academy_id')->constrained('organizations');
+            $table->unsignedBigInteger('academy_id')->index();
             $table->string('wa_message_id')->nullable()->index();
             $table->string('phone_number', 20);
             $table->string('type', 30);
@@ -20,7 +20,7 @@ public function up(): void
             $table->text('error_message')->nullable();
             $table->jsonb('payload')->default('{}');
             $table->jsonb('response')->default('{}');
-            $table->foreignId('sent_by')->nullable()->constrained('users');
+            $table->unsignedBigInteger('sent_by')->nullable()->index();
             $table->timestamp('sent_at')->nullable();
             $table->timestamp('delivered_at')->nullable();
             $table->timestamp('read_at')->nullable();