IT Assets: add Maintenance page with modal form

New database table: maintenance_records
- Fields: asset, start_date, end_date, status (open/in_progress/closed),
  initial_diagnosis, action_taken, action_result

New routes (/maintenance/):
- List with status + asset filters and pagination
- POST /new — create record (editor+); auto-sets asset status to 'maintenance'
- GET /<id>/edit — opens list page with modal pre-filled
- POST /<id>/update — save edits; auto-clears 'maintenance' status on close
- POST /<id>/delete — remove record

UI:
- Bootstrap modal form shared for add and edit
- Clicking any table row opens the modal pre-filled with that row's data
- Asset field locked in edit mode (cannot change asset)
- Delete button per row (with confirmation)
- Sidebar: 'Maintenance' link added under Hardware section

IT_asset_management/app/templates/base.html

@@ -139,6 +139,10 @@
      class="nav-link {% if request.blueprint == 'assets' %}active{% endif %}">
     <i class="bi bi-laptop"></i> Assets
   </a>
+  <a href="{{ url_for('maintenance.index') }}"
+     class="nav-link {% if request.blueprint == 'maintenance' %}active{% endif %}">
+    <i class="bi bi-wrench-adjustable"></i> Maintenance
+  </a>
   {% if current_user.is_editor %}
   <a href="{{ url_for('assets.create') }}"
      class="nav-link {% if request.endpoint == 'assets.create' %}active{% endif %}">