feat: M2 database migration runner with Phinx and initial schema

db/migrations/20261011120001_create_folders.php

@@ -0,0 +1,31 @@
+<?php
+
+declare(strict_types=1);
+
+use Phinx\Migration\AbstractMigration;
+
+/**
+ * Initial schema: folders mirroring the managed library tree.
+ *
+ * Folders form a recursive tree structure where each folder has an optional
+ * parent folder. The root folder has parent_id = NULL. The path is stored
+ * as a materialized absolute path from the library root for efficient queries.
+ */
+final class CreateFolders extends AbstractMigration
+{
+    public function change(): void
+    {
+        $this->table('folders', ['id' => 'id', 'primary_key' => 'id'])
+            ->addColumn('parent_id', 'integer', ['null' => true, 'signed' => false])
+            ->addColumn('name', 'string', ['limit' => 255])
+            ->addColumn('path', 'text', ['null' => false])
+            ->addColumn('added_at', 'timestamp', ['default' => 'CURRENT_TIMESTAMP', 'timezone' => true])
+            ->addForeignKey('parent_id', 'folders', 'id', ['delete' => 'CASCADE', 'update' => 'NO_ACTION'])
+            ->addIndex(['parent_id', 'name'], ['unique' => true, 'name' => 'idx_folders_parent_name'])
+            ->addIndex(['path'], ['unique' => true, 'name' => 'idx_folders_path'])
+            ->create();
+
+        // Root folder entry (will be created by application on first run)
+        // Application ensures root folder exists with path = '/' and parent_id = NULL
+    }
+}