feat: dodaj prefill email potpisa i body logging

Uvodi signature polje i default predlozak potpisa za nove korisnike, te data migraciju koja popunjava postojece prazne potpise. Email flow sada dosljedno dodaje korisnicki potpis i sprema finalni body poruke u dispatch log radi audita.\n\nCo-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/modules/fleet/views.py

@@ -30,6 +30,7 @@ from reportlab.lib.styles import getSampleStyleSheet
 from reportlab.pdfgen import canvas
 from reportlab.platypus import Table, TableStyle, Paragraph
 from .pdf_layout import register_unicode_fonts, draw_standard_header_footer
+from .email_utils import append_user_signature
 from rest_framework import viewsets, permissions, status, mixins
 from rest_framework.decorators import action, api_view, permission_classes
 from rest_framework.response import Response
@@ -2203,10 +2204,11 @@ class VehicleServiceRecordViewSet(viewsets.ModelViewSet):
         body = _first_non_empty(request.data.get('message')) or (
             f"U prilogu je PDF servisnog zapisa. Poslano {timezone.now().strftime('%d.%m.%Y %H:%M')}."
         )
+        body_with_signature = append_user_signature(body, request.user)
         _send_document_email(
             recipient=recipient,
             subject=subject,
-            body=body,
+            body=body_with_signature,
             filename=f"{service_record.pk}.service-record.pdf",
             pdf_bytes=pdf_bytes,
         )
@@ -2216,7 +2218,7 @@ class VehicleServiceRecordViewSet(viewsets.ModelViewSet):
             service_record=service_record,
             recipients=[recipient],
             subject=subject,
-            message=body,
+            message=body_with_signature,
             attachments=[{'type': 'service_record_pdf', 'filename': f"{service_record.pk}.service-record.pdf"}],
             status='sent',
             sent_at=timezone.now(),