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>

CLAUDE.md

@@ -234,7 +234,8 @@ wp-prometheus/
 │   ├── Metrics/
 │   │   ├── Collector.php        # Prometheus metrics collector
 │   │   ├── CustomMetricBuilder.php # Custom metric CRUD
-│   │   └── RuntimeCollector.php # Runtime metrics collector
+│   │   ├── RuntimeCollector.php # Runtime metrics collector
+│   │   └── StorageFactory.php   # Storage adapter factory
 │   ├── Installer.php         # Activation/Deactivation
 │   ├── Plugin.php            # Main plugin class
 │   └── index.php
@@ -290,6 +291,34 @@ add_action( 'wp_prometheus_collect_metrics', function( $collector ) {
 
 ## Session History
 
+### 2026-02-02 - Persistent Storage (v0.4.0)
+
+- Added persistent storage support for metrics:
+  - `StorageFactory.php` - Factory class for storage adapter instantiation
+  - Redis storage adapter for shared metrics across multiple instances
+  - APCu storage adapter for single-server high-performance caching
+  - Automatic fallback to In-Memory if configured adapter fails
+- Added new "Storage" tab in admin settings:
+  - Storage adapter selection (In-Memory, Redis, APCu)
+  - Redis configuration (host, port, password, database, key prefix)
+  - APCu configuration (key prefix)
+  - Connection test button with detailed error messages
+- Added environment variable support for Docker deployments:
+  - `WP_PROMETHEUS_STORAGE_ADAPTER` - Adapter selection
+  - `WP_PROMETHEUS_REDIS_HOST`, `_PORT`, `_PASSWORD`, `_DATABASE`, `_PREFIX`
+  - `WP_PROMETHEUS_APCU_PREFIX`
+  - Environment variables take precedence over admin settings
+- Updated `Collector.php` to use `StorageFactory::get_adapter()`
+- Updated Help tab with storage backends documentation
+- Updated translation files with all new strings
+- **Key Learning**: promphp/prometheus_client_php storage adapters
+  - Redis adapter requires options array with host, port, password, timeout
+  - APCu adapter just needs a prefix string
+  - Use `Redis::setPrefix()` before instantiation for custom key prefixes
+- **Key Learning**: Docker environment variable configuration
+  - Use `getenv()` with explicit false check (`false !== getenv()`)
+  - Environment variables should override WordPress options for containerized deployments
+
 ### 2026-02-02 - Custom Metrics & Dashboards (v0.3.0)
 
 - Added Custom Metric Builder with full admin UI: