feat: initial WooCow plugin — Mailcow/WooCommerce integration

- Mailcow API client wrapping domains, mailboxes, aliases endpoints
- Admin backend: server management, customer-domain assignments, mailbox overview
- WooCommerce My Account: email hosting tab with mailbox/alias management
- Per-mailbox password change (independent of WP account password)
- Optional WP account password sync to all customer mailboxes
- Installer creates wp_woocow_servers and wp_woocow_assignments DB tables
- Full nonce + capability + ownership verification on all AJAX endpoints

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

includes/class-woocow-installer.php

@@ -0,0 +1,39 @@
+<?php
+defined( 'ABSPATH' ) || exit;
+
+class WooCow_Installer {
+
+    public static function install(): void {
+        global $wpdb;
+
+        $charset = $wpdb->get_charset_collate();
+
+        $servers_sql = "CREATE TABLE {$wpdb->prefix}woocow_servers (
+            id         INT NOT NULL AUTO_INCREMENT,
+            name       VARCHAR(100) NOT NULL,
+            url        VARCHAR(500) NOT NULL,
+            api_key    VARCHAR(500) NOT NULL,
+            active     TINYINT(1) NOT NULL DEFAULT 1,
+            created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+            PRIMARY KEY (id)
+        ) $charset;";
+
+        $assignments_sql = "CREATE TABLE {$wpdb->prefix}woocow_assignments (
+            id          INT NOT NULL AUTO_INCREMENT,
+            customer_id BIGINT NOT NULL,
+            server_id   INT NOT NULL,
+            domain      VARCHAR(255) NOT NULL,
+            created_at  DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+            PRIMARY KEY (id),
+            KEY         idx_customer (customer_id),
+            KEY         idx_server   (server_id),
+            UNIQUE KEY  uniq_cust_domain (customer_id, domain)
+        ) $charset;";
+
+        require_once ABSPATH . 'wp-admin/includes/upgrade.php';
+        dbDelta( $servers_sql );
+        dbDelta( $assignments_sql );
+
+        update_option( 'woocow_db_version', WOOCOW_VERSION );
+    }
+}