feat(attachments): create Attachment model and migration for file attachments

database/migrations/2026_06_25_000500_create_attachments_table.php

@@ -0,0 +1,40 @@
+<?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('attachments', function (Blueprint $table) {
+            $table->id();
+            $table->string('attachable_type');
+            $table->unsignedBigInteger('attachable_id');
+            $table->string('path');
+            $table->string('filename');
+            $table->string('type')->nullable();
+            $table->string('mime_type');
+            $table->boolean('is_public')->default(false);
+            $table->string('extension', 20)->nullable();
+            $table->unsignedBigInteger('size')->default(0);
+            $table->timestamps();
+
+            $table->index(['attachable_type', 'attachable_id']);
+            $table->index('type');
+            $table->index('mime_type');
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::dropIfExists('attachments');
+    }
+};