feat: Add Prometheus metrics integration

- Add includes/Prometheus/Integration.php with metrics collection
- Expose content, engagement, user, WooCommerce, and ActivityPub metrics
- Add settings toggle in Integrations tab
- Requires WP Prometheus plugin to be active
- Bump version to 0.5.0

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

wp-fedistream.php

@@ -3,7 +3,7 @@
  * Plugin Name: WP FediStream
  * Plugin URI: https://src.bundespruefstelle.ch/magdev/wp-fedistream
  * Description: Stream music over ActivityPub - Build your own music streaming platform for Musicians and Labels.
- * Version: 0.4.9
+ * Version: 0.5.0
  * Requires at least: 6.4
  * Requires PHP: 8.3
  * Author: Marco Graetsch
@@ -26,7 +26,7 @@ if ( ! defined( 'ABSPATH' ) ) {
  *
  * @var string
  */
-define( 'WP_FEDISTREAM_VERSION', '0.4.9' );
+define( 'WP_FEDISTREAM_VERSION', '0.5.0' );
 
 /**
  * Plugin file path.