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>

CHANGELOG.md

@@ -7,6 +7,19 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## [0.5.0] - 2026-02-02
+
+### Added
+
+- **Prometheus Metrics Integration** - Expose FediStream metrics for monitoring
+  - Content metrics: `fedistream_content_total` (by type/status), `fedistream_genres_total`, `fedistream_moods_total`
+  - Engagement metrics: `fedistream_plays_total`, `fedistream_plays_today`, `fedistream_favorites_total`, `fedistream_local_follows_total`, `fedistream_listening_history_entries`
+  - User metrics: `fedistream_users_with_library`, `fedistream_users_following_artists`, `fedistream_notifications_total`, `fedistream_notifications_pending`
+  - WooCommerce metrics (conditional): `fedistream_purchases_total`, `fedistream_customers_total`, `fedistream_products_total`
+  - ActivityPub metrics (conditional): `fedistream_activitypub_followers_total`, `fedistream_activitypub_followers_by_artist`, `fedistream_activitypub_reactions_total`
+  - New setting in Integrations tab to enable/disable Prometheus metrics
+  - Requires WP Prometheus plugin to be active
+
 ## [0.4.9] - 2026-02-02
 
 ### Changed