refactor(attachments): remove is_public field from Attachment model and migration

database/migrations/2026_06_25_000500_create_attachments_table.php

@@ -19,7 +19,6 @@ return new class extends Migration
             $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();