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>

includes/Installer.php

@@ -347,6 +347,7 @@ class Installer {
         $defaults = array(
             'wp_fedistream_enable_activitypub'       => 1,
             'wp_fedistream_enable_woocommerce'       => 0,
+            'wp_fedistream_enable_prometheus'        => 0,
             'wp_fedistream_audio_formats'            => array( 'mp3', 'wav', 'flac', 'ogg' ),
             'wp_fedistream_max_upload_size'          => 50, // MB
             'wp_fedistream_default_license'          => 'all-rights-reserved',