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>

languages/wp-prometheus.pot

@@ -2,7 +2,7 @@
 # This file is distributed under the GPL v2 or later.
 msgid ""
 msgstr ""
-"Project-Id-Version: WP Prometheus 0.3.0\n"
+"Project-Id-Version: WP Prometheus 0.4.0\n"
 "Report-Msgid-Bugs-To: https://src.bundespruefstelle.ch/magdev/wp-prometheus/issues\n"
 "POT-Creation-Date: 2026-02-02T00:00:00+00:00\n"
 "MIME-Version: 1.0\n"
@@ -614,3 +614,265 @@ msgstr ""
 #: wp-prometheus.php
 msgid "WP Prometheus requires PHP version %s or higher."
 msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Storage"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Metrics Storage Configuration"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Configure how Prometheus metrics are stored. Persistent storage (Redis, APCu) allows metrics to survive between requests and aggregate data over time."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Environment Override Active"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Storage adapter is configured via environment variable. Admin settings will be ignored."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Storage Fallback Active"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Falling back to In-Memory storage."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Current Status:"
+msgstr ""
+
+#. translators: %s: Active adapter name
+#: src/Admin/Settings.php
+msgid "Using %s storage."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Storage Adapter"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "unavailable"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Select the storage backend for metrics. Redis and APCu require their respective PHP extensions."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Redis Configuration"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Host"
+msgstr ""
+
+#. translators: %s: Environment variable name
+#: src/Admin/Settings.php
+msgid "Can be overridden with %s environment variable."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Port"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Password"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Leave empty if not required"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Database"
+msgstr ""
+
+#. translators: %s: Environment variable name
+#: src/Admin/Settings.php
+msgid "Redis database index (0-15). Can be overridden with %s."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Key Prefix"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Prefix for Redis keys. Useful when sharing Redis with other applications."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "APCu Configuration"
+msgstr ""
+
+#. translators: %s: Environment variable name
+#: src/Admin/Settings.php
+msgid "Prefix for APCu keys. Can be overridden with %s."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Save Storage Settings"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Test Connection"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Environment Variables"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "For Docker or containerized environments, you can configure storage using environment variables. These take precedence over admin settings."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Variable"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Example"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Storage adapter to use"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Redis server hostname"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Redis server port"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Redis authentication password"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Redis database index"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Redis key prefix"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "APCu key prefix"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Docker Compose Example"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Permission denied."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Storage adapter is configured via environment variable and cannot be changed."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Invalid storage adapter."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Storage settings saved successfully."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Storage settings saved, but connection test failed:"
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "In-Memory (default, no persistence)"
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "Redis (requires PHP Redis extension)"
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "APCu (requires APCu extension)"
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "PHP Redis extension is not installed."
+msgstr ""
+
+#. translators: %s: Error message
+#: src/Metrics/StorageFactory.php
+msgid "Redis connection failed: %s"
+msgstr ""
+
+#. translators: %s: Error message
+#: src/Metrics/StorageFactory.php
+msgid "Redis error: %s"
+msgstr ""
+
+#. translators: %s: Error message
+#: src/Metrics/StorageFactory.php
+msgid "Storage error: %s"
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "APCu extension is not installed."
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "APCu is installed but not enabled."
+msgstr ""
+
+#. translators: %s: Error message
+#: src/Metrics/StorageFactory.php
+msgid "APCu error: %s"
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "In-Memory storage is always available."
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "Unknown storage adapter."
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "Could not connect to Redis server."
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "Redis authentication failed."
+msgstr ""
+
+#. translators: %s: Redis host:port
+#: src/Metrics/StorageFactory.php
+msgid "Successfully connected to Redis at %s."
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "Redis ping failed."
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "APCu is installed but not enabled. Check your php.ini settings."
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "APCu store operation failed."
+msgstr ""
+
+#. translators: %s: Memory info
+#: src/Metrics/StorageFactory.php
+msgid "APCu is working. Memory: %s used."
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "APCu fetch operation returned unexpected value."
+msgstr ""