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>

src/Metrics/Collector.php

@@ -8,9 +8,9 @@
 namespace Magdev\WpPrometheus\Metrics;
 
 use Prometheus\CollectorRegistry;
-use Prometheus\Storage\InMemory;
 use Prometheus\RenderTextFormat;
 use Magdev\WpPrometheus\Metrics\CustomMetricBuilder;
+use Magdev\WpPrometheus\Metrics\StorageFactory;
 
 // Prevent direct file access.
 if ( ! defined( 'ABSPATH' ) ) {
@@ -42,7 +42,7 @@ class Collector {
      * Constructor.
      */
     public function __construct() {
-        $this->registry = new CollectorRegistry( new InMemory() );
+        $this->registry = new CollectorRegistry( StorageFactory::get_adapter() );
     }
 
     /**