Add Update Manager for automated Part-DB updates

This feature adds a comprehensive Update Manager similar to Mainsail's
update system, allowing administrators to update Part-DB directly from
the web interface.

Features:
- Web UI at /admin/update-manager showing current and available versions
- Support for Git-based installations with automatic update execution
- Maintenance mode during updates to prevent user access
- Automatic database backup before updates
- Git rollback points for recovery (tags created before each update)
- Progress tracking with real-time status updates
- Update history and log viewing
- Downgrade support with appropriate UI messaging
- CLI command `php bin/console partdb:update` for server-side updates

New files:
- UpdateManagerController: Handles all web UI routes
- UpdateCommand: CLI command for running updates
- UpdateExecutor: Core update execution logic with safety mechanisms
- UpdateChecker: GitHub API integration for version checking
- InstallationTypeDetector: Detects installation type (Git/Docker/ZIP)
- MaintenanceModeSubscriber: Blocks user access during maintenance
- UpdateExtension: Twig functions for update notifications

UI improvements:
- Update notification in navbar for admins when update available
- Confirmation dialogs for update/downgrade actions
- Downgrade-specific text throughout the interface
- Progress page with auto-refresh

src/Twig/UpdateExtension.php

@@ -0,0 +1,79 @@
+<?php
+/*
+ * This file is part of Part-DB (https://github.com/Part-DB/Part-DB-symfony).
+ *
+ *  Copyright (C) 2019 - 2024 Jan Böhmer (https://github.com/jbtronics)
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU Affero General Public License as published
+ *  by the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU Affero General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Affero General Public License
+ *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+declare(strict_types=1);
+
+
+namespace App\Twig;
+
+use App\Services\System\UpdateAvailableManager;
+use Symfony\Bundle\SecurityBundle\Security;
+use Twig\Extension\AbstractExtension;
+use Twig\TwigFunction;
+
+/**
+ * Twig extension for update-related functions.
+ */
+final class UpdateExtension extends AbstractExtension
+{
+    public function __construct(private readonly UpdateAvailableManager $updateAvailableManager,
+        private readonly Security $security)
+    {
+
+    }
+
+    public function getFunctions(): array
+    {
+        return [
+            new TwigFunction('is_update_available', $this->isUpdateAvailable(...)),
+            new TwigFunction('get_latest_version', $this->getLatestVersion(...)),
+            new TwigFunction('get_latest_version_url', $this->getLatestVersionUrl(...)),
+        ];
+    }
+
+    /**
+     * Check if an update is available and the user has permission to see it.
+     */
+    public function isUpdateAvailable(): bool
+    {
+        // Only show to users with the show_updates permission
+        if (!$this->security->isGranted('@system.show_updates')) {
+            return false;
+        }
+
+        return $this->updateAvailableManager->isUpdateAvailable();
+    }
+
+    /**
+     * Get the latest available version string.
+     */
+    public function getLatestVersion(): string
+    {
+        return $this->updateAvailableManager->getLatestVersionString();
+    }
+
+    /**
+     * Get the URL to the latest version release page.
+     */
+    public function getLatestVersionUrl(): string
+    {
+        return $this->updateAvailableManager->getLatestVersionUrl();
+    }
+}