feat: async email slanje s cache PDF fallbackom i admin logom

Prebaci slanje emaila putnog naloga u pozadinski task bez blokiranja UI-a i dodaj fallback kad queue nije dostupna. Kod greške slanja spremi generirane PDF-ove u cache i pošalji notifikaciju s download linkovima.

Dodaj EmailDispatchLog model i Django admin pregled za praćenje kome je poslano, što je poslano, status i detalje privitaka.

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

backend/modules/fleet/admin.py

@@ -10,6 +10,7 @@ from .models import (
     WorkOrderAdditionalCostsTable,
     ServiceContextNote,
     GeneratedWorkOrderPdf,
+    EmailDispatchLog,
     VehicleServiceRecord,
     VehicleServicePhoto,
     VehicleServiceAttachment,
@@ -244,4 +245,50 @@ class VehicleServiceRecordAdmin(admin.ModelAdmin):
     def short_description(self, obj):
         if obj.description:
             return obj.description[:60] + ('…' if len(obj.description) > 60 else '')
-        return "—"
+        return "—"
+
+
+@admin.register(EmailDispatchLog)
+class EmailDispatchLogAdmin(admin.ModelAdmin):
+    list_display = (
+        'id',
+        'dispatch_type',
+        'status',
+        'requested_by',
+        'work_order',
+        'service_record',
+        'recipients_count',
+        'sent_at',
+        'created_at',
+    )
+    list_filter = ('dispatch_type', 'status', 'created_at', 'sent_at')
+    search_fields = (
+        'subject',
+        'message',
+        'error_message',
+        'requested_by__email',
+        'requested_by__username',
+        'work_order__display_code',
+    )
+    readonly_fields = (
+        'dispatch_type',
+        'requested_by',
+        'work_order',
+        'service_record',
+        'recipients',
+        'subject',
+        'message',
+        'attachments',
+        'status',
+        'error_message',
+        'metadata',
+        'sent_at',
+        'created_at',
+        'updated_at',
+    )
+    ordering = ('-created_at',)
+
+    @admin.display(description='Primatelja')
+    def recipients_count(self, obj):
+        recipients = obj.recipients or []
+        return len(recipients) if isinstance(recipients, list) else 0