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

templates/admin/update_manager/log_viewer.html.twig

@@ -0,0 +1,40 @@
+{% extends "main_card.html.twig" %}
+
+{% block title %}{{ filename }} - {% trans %}update_manager.log_viewer{% endtrans %}{% endblock %}
+
+{% block card_title %}
+    <i class="fas fa-file-code"></i> {{ filename }}
+{% endblock %}
+
+{% block card_content %}
+<div class="mb-4">
+    <a href="{{ path('admin_update_manager') }}" class="btn btn-outline-secondary">
+        <i class="fas fa-arrow-left me-1"></i> {% trans %}update_manager.back_to_update_manager{% endtrans %}
+    </a>
+</div>
+
+<div class="card">
+    <div class="card-header d-flex justify-content-between align-items-center">
+        <span>
+            <i class="fas fa-terminal me-2"></i>{% trans %}update_manager.update_log{% endtrans %}
+        </span>
+        <span class="badge bg-secondary">{{ content|length }} {% trans %}update_manager.bytes{% endtrans %}</span>
+    </div>
+    <div class="card-body p-0">
+        <pre class="bg-dark text-light p-3 mb-0" style="max-height: 600px; overflow-y: auto; white-space: pre-wrap; word-break: break-all;"><code>{{ content }}</code></pre>
+    </div>
+</div>
+
+<style>
+    pre code {
+        font-family: 'SF Mono', 'Consolas', 'Monaco', monospace;
+        font-size: 0.85rem;
+        line-height: 1.5;
+    }
+
+    /* Highlight different log levels */
+    pre code {
+        color: #e0e0e0;
+    }
+</style>
+{% endblock %}