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>

CHANGELOG.md

@@ -5,6 +5,38 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.4.0] - 2026-02-02
+
+### Added
+
+- Persistent Storage Support:
+  - Redis storage adapter for shared metrics across multiple instances
+  - APCu storage adapter for single-server high-performance caching
+  - StorageFactory class for automatic adapter selection and fallback
+  - Connection testing with detailed error messages
+- 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
+  - Environment variables documentation
+- Environment variable configuration for Docker/containerized environments:
+  - `WP_PROMETHEUS_STORAGE_ADAPTER` - Select storage adapter
+  - `WP_PROMETHEUS_REDIS_HOST` - Redis server hostname
+  - `WP_PROMETHEUS_REDIS_PORT` - Redis server port
+  - `WP_PROMETHEUS_REDIS_PASSWORD` - Redis authentication
+  - `WP_PROMETHEUS_REDIS_DATABASE` - Redis database index (0-15)
+  - `WP_PROMETHEUS_REDIS_PREFIX` - Redis key prefix
+  - `WP_PROMETHEUS_APCU_PREFIX` - APCu key prefix
+- Automatic fallback to In-Memory storage if configured adapter fails
+- Docker Compose example in admin settings
+
+### Changed
+
+- Settings page now has 6 tabs: License, Metrics, Storage, Custom Metrics, Dashboards, Help
+- Updated translations with all new strings (English and German)
+- Collector now uses StorageFactory for storage adapter instantiation
+
 ## [0.3.0] - 2026-02-02
 
 ### Added