feat: Add persistent storage support for Redis and APCu (v0.4.0)

- Add StorageFactory class for storage adapter selection with fallback
- Support Redis storage for shared metrics across instances
- Support APCu storage for high-performance single-server deployments
- Add Storage tab in admin settings with configuration UI
- Add connection testing for Redis and APCu adapters
- Support environment variables for Docker/containerized deployments
- Update Collector to use StorageFactory instead of hardcoded InMemory
- Add all translations (English and German)

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

wp-prometheus.php

@@ -3,7 +3,7 @@
  * Plugin Name: WP Prometheus
  * Plugin URI: https://src.bundespruefstelle.ch/magdev/wp-prometheus
  * Description: Prometheus metrics endpoint for WordPress with extensible hooks for custom metrics.
- * Version: 0.3.0
+ * Version: 0.4.0
  * Requires at least: 6.4
  * Requires PHP: 8.3
  * Author: Marco Graetsch
@@ -26,7 +26,7 @@ if ( ! defined( 'ABSPATH' ) ) {
  *
  * @var string
  */
-define( 'WP_PROMETHEUS_VERSION', '0.3.0' );
+define( 'WP_PROMETHEUS_VERSION', '0.4.0' );
 
 /**
  * Plugin file path.