feat: add background tasks monitoring dashboard

Adds a unified view of all background tasks (imports and test runs)
under Platform Monitoring. Includes real-time status polling,
statistics overview, and task history.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/templates/admin/partials/sidebar.html

@@ -103,6 +103,7 @@
     <!-- Platform Monitoring Section -->
     {{ section_header('Platform Monitoring', 'monitoring') }}
     {% call section_content('monitoring') %}
+        {{ menu_item('background-tasks', '/admin/background-tasks', 'collection', 'Background Tasks') }}
         {{ menu_item('imports', '/admin/imports', 'cube', 'Import Jobs') }}
         {{ menu_item('logs', '/admin/logs', 'document-text', 'Application Logs') }}
     {% endcall %}