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>

PLAN.md

@@ -59,6 +59,7 @@ wp-prometheus/
 │   └── release.yml          # CI/CD pipeline
 ├── assets/
 │   ├── css/                  # Admin/Frontend styles
+│   ├── dashboards/           # Grafana dashboard templates
 │   └── js/
 │       └── admin.js          # Admin JavaScript
 ├── languages/                # Translation files
@@ -67,13 +68,17 @@ wp-prometheus/
 ├── releases/                 # Release packages
 ├── src/
 │   ├── Admin/
+│   │   ├── DashboardProvider.php
 │   │   └── Settings.php
 │   ├── Endpoint/
 │   │   └── MetricsEndpoint.php
 │   ├── License/
 │   │   └── Manager.php
 │   ├── Metrics/
-│   │   └── Collector.php
+│   │   ├── Collector.php
+│   │   ├── CustomMetricBuilder.php
+│   │   ├── RuntimeCollector.php
+│   │   └── StorageFactory.php
 │   ├── Installer.php
 │   ├── Plugin.php
 │   └── index.php
@@ -159,13 +164,57 @@ Alternatively, the token can be passed as a query parameter (for testing):
 https://example.com/metrics/?token=your-auth-token
 ```
 
+## Storage Configuration
+
+The plugin supports multiple storage backends for Prometheus metrics:
+
+### Available Adapters
+
+| Adapter   | Description                     | Use Case                              |
+| --------- | ------------------------------- | ------------------------------------- |
+| In-Memory | Default, no persistence         | Development, single request metrics   |
+| Redis     | Shared storage across instances | Production, load-balanced environments|
+| APCu      | High-performance local cache    | Production, single-server deployments |
+
+### Environment Variables
+
+For Docker or containerized environments, configure storage via environment variables:
+
+```bash
+# Storage adapter selection
+WP_PROMETHEUS_STORAGE_ADAPTER=redis
+
+# Redis configuration
+WP_PROMETHEUS_REDIS_HOST=redis
+WP_PROMETHEUS_REDIS_PORT=6379
+WP_PROMETHEUS_REDIS_PASSWORD=secret
+WP_PROMETHEUS_REDIS_DATABASE=0
+WP_PROMETHEUS_REDIS_PREFIX=WORDPRESS_PROMETHEUS_
+
+# APCu configuration
+WP_PROMETHEUS_APCU_PREFIX=wp_prom
+```
+
+### Docker Compose Example
+
+```yaml
+services:
+  wordpress:
+    image: wordpress:latest
+    environment:
+      WP_PROMETHEUS_STORAGE_ADAPTER: redis
+      WP_PROMETHEUS_REDIS_HOST: redis
+      WP_PROMETHEUS_REDIS_PORT: 6379
+    depends_on:
+      - redis
+
+  redis:
+    image: redis:alpine
+```
+
 ## Future Enhancements
 
-### Version 0.3.0
-
-- Custom metric builder in admin
-- Metric export/import
-- Grafana dashboard templates
+*No planned features at this time.*
 
 ## Dependencies