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/tasks.py

@@ -29,6 +29,7 @@ from reportlab.lib.utils import ImageReader
 from reportlab.pdfgen import canvas
 from .models import VehicleNotification, GeneratedWorkOrderPdf
 from .pdf_layout import register_unicode_fonts, draw_standard_header_footer
+from .email_utils import append_user_signature
 
 logger = logging.getLogger(__name__)
 register_unicode_fonts()
@@ -435,7 +436,7 @@ def send_work_order_email_bundle_task(
 
     email_message = EmailMessage(
         subject=subject or f"Putni nalog {_work_order_display_code(work_order)}",
-        body="\n".join(body_lines).strip(),
+        body=append_user_signature("\n".join(body_lines).strip(), requested_by),
         from_email=from_email,
         to=recipients or [],
     )
@@ -450,7 +451,8 @@ def send_work_order_email_bundle_task(
             }
             for filename, content, content_type in attachments
         ]
-        dispatch_log.save(update_fields=['attachments', 'updated_at'])
+        dispatch_log.message = email_message.body
+        dispatch_log.save(update_fields=['attachments', 'message', 'updated_at'])
 
     try:
         email_message.send(fail_silently=False)