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/fleet/models.py

@@ -565,4 +565,72 @@ class VehicleNotification(BaseModel):
         ordering = ['-created_at']
 
     def __str__(self):
-        return f"{self.title} -> {self.recipient} ({self.level})"
+        return f"{self.title} -> {self.recipient} ({self.level})"
+
+
+class ServiceContextNote(BaseModel):
+    AUDIENCE_SOURCE_CHOICES = [
+        ('self', 'Samo autor'),
+        ('member', 'Odabrani član tima'),
+        ('team', 'Svi članovi tima'),
+    ]
+
+    created_by = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        on_delete=models.CASCADE,
+        related_name='created_service_context_notes',
+        verbose_name=_("Kreirao"),
+    )
+    recipient = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        on_delete=models.CASCADE,
+        related_name='service_context_notes',
+        verbose_name=_("Primatelj"),
+    )
+    audience_source = models.CharField(
+        max_length=16,
+        choices=AUDIENCE_SOURCE_CHOICES,
+        default='self',
+        verbose_name=_("Publika"),
+    )
+    work_order = models.ForeignKey(
+        WorkOrder,
+        on_delete=models.SET_NULL,
+        null=True,
+        blank=True,
+        related_name='service_context_notes',
+        verbose_name=_("Putni nalog"),
+    )
+    task = models.ForeignKey(
+        'task_management.Task',
+        on_delete=models.SET_NULL,
+        null=True,
+        blank=True,
+        related_name='service_context_notes',
+        verbose_name=_("Radni zadatak"),
+    )
+    note = models.TextField(verbose_name=_("Bilješka"))
+    note_date = models.DateField(
+        null=True,
+        blank=True,
+        db_index=True,
+        verbose_name=_("Datum podsjetnika"),
+    )
+    is_closed = models.BooleanField(default=False, verbose_name=_("Ručno zatvoreno"))
+    closed_at = models.DateTimeField(null=True, blank=True, verbose_name=_("Vrijeme zatvaranja"))
+
+    class Meta:
+        verbose_name = _("Bilješka servisnog konteksta")
+        verbose_name_plural = _("Bilješke servisnog konteksta")
+        ordering = ['is_closed', 'note_date', '-created_at']
+
+    def __str__(self):
+        return f"Note {self.pk} -> {self.recipient_id}"
+
+    def clean(self):
+        super().clean()
+        if self.task_id and self.work_order_id and self.task and self.task.work_order_id:
+            if self.task.work_order_id != self.work_order_id:
+                raise ValidationError({
+                    'task': _("Radni zadatak mora biti povezan s istim putnim nalogom kao bilješka."),
+                })