feat: dodaj bilješke widget i admin kalendar/notifikacije

Uveden je novi Notes drawer s kalendar integracijom, zajednički DRY
left-slide mehanizam te međusobno isključivo prikazivanje s Kalendar
widgetom.

Backend i frontend su prošireni za servisne bilješke (uključujući slanje
na više servisera), dodan je users/servicers endpoint, te su uvedene
warning notifikacije kada serviser ima više od 2 zadatka isti dan.

U admin su dodane stavke za Bilješke i zaseban Kalendar zadataka (proxy
TaskCalendar) radi lakšeg operativnog pregleda.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/modules/task_management/admin.py

@@ -2,7 +2,7 @@
 
 from django.contrib import admin
 
-from .models import Task, TaskTemplate, TaskTemplateEntry, TaskWorkEntry, TaskWorkHoursTable
+from .models import Task, TaskCalendar, TaskTemplate, TaskTemplateEntry, TaskWorkEntry, TaskWorkHoursTable
 
 
 class TaskWorkEntryInline(admin.TabularInline):
@@ -33,6 +33,19 @@ class TaskAdmin(admin.ModelAdmin):
     inlines = (TaskWorkHoursTableInline, TaskWorkEntryInline)
 
 
+@admin.register(TaskCalendar)
+class TaskCalendarAdmin(admin.ModelAdmin):
+    list_display = ('scheduled_date', 'title', 'status', 'assigned_to', 'vehicle', 'work_order')
+    list_filter = ('scheduled_date', 'status', 'assigned_to')
+    search_fields = ('title', 'assigned_to__email', 'vehicle__registration_number', 'work_order__display_code')
+    readonly_fields = ('id', 'created_at', 'updated_at')
+    ordering = ('scheduled_date', 'assigned_to__first_name', 'assigned_to__last_name')
+
+    def get_queryset(self, request):
+        queryset = super().get_queryset(request)
+        return queryset.filter(is_active=True, scheduled_date__isnull=False)
+
+
 @admin.register(TaskTemplate)
 class TaskTemplateAdmin(admin.ModelAdmin):
     list_display = ('code', 'title', 'status', 'is_active', 'created_at')