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/core/users/models.py

@@ -3,6 +3,17 @@ from django.core.exceptions import ValidationError
 from django.db import models
 from .managers import CustomUserManager
 
+DEFAULT_EMAIL_SIGNATURE_TEMPLATE = (
+    "Poslano iz aplikacije\n"
+    "S poštovanjem / Freundliche Grüße / Kind regards\n\n"
+    "{name_line}\n\n"
+    "KNEZ LJUBO d.o.o.\n"
+    "Ivlje 44, 10040 Zagreb\n\n"
+    "Office: Donje Svetice 40, 10000 Zagreb\n\n"
+    "Mobile: {mobile}\n\n"
+    "www.knezljubo.hr"
+)
+
 class CustomUser(AbstractUser):
     # Email koristimo za login, pa mora biti jedinstven
     username=None
@@ -19,6 +30,7 @@ class CustomUser(AbstractUser):
     occupation = models.CharField(max_length=120, blank=True, default='', verbose_name="Zanimanje")
     residence = models.CharField(max_length=255, blank=True, default='', verbose_name="Prebivalište")
     work_position = models.CharField(max_length=120, blank=True, default='', verbose_name="Na radnom mjestu")
+    signature = models.TextField(blank=True, default='', verbose_name="Email potpis")
     licence_number = models.CharField(max_length=50, blank=True, null=True, verbose_name="Broj licence")
     is_verified = models.BooleanField(default=False, verbose_name="Verificiran profil")
 
@@ -47,6 +59,27 @@ class CustomUser(AbstractUser):
         if self.is_kupac and not self.client_profile:
             raise ValidationError({'client_profile': "Korisnik koji je označen kao kupac mora imati povezan klijentski profil."})
 
+    def build_default_signature(self):
+        full_name = self.get_full_name().strip()
+        fallback_name = self.email.strip() if isinstance(self.email, str) and self.email.strip() else 'Ime i prezime'
+        display_name = full_name or fallback_name
+        occupation = self.occupation.strip() if isinstance(self.occupation, str) else ''
+        name_line = f"{occupation} {display_name}".strip() if occupation else display_name
+        mobile = self.telefon.strip() if isinstance(self.telefon, str) and self.telefon.strip() else "+385 91 636 3466"
+        return DEFAULT_EMAIL_SIGNATURE_TEMPLATE.format(
+            name_line=name_line,
+            mobile=mobile,
+        )
+
+    def get_email_signature(self):
+        stored_signature = self.signature.strip() if isinstance(self.signature, str) else ''
+        if stored_signature:
+            return stored_signature
+        return self.build_default_signature()
+
     def save(self, *args, **kwargs):
+        signature = self.signature.strip() if isinstance(self.signature, str) else ''
+        if not signature:
+            self.signature = self.build_default_signature()
         self.full_clean() # Prisiljava pokretanje clean() prije svakog save()
-        super().save(*args, **kwargs)
+        super().save(*args, **kwargs)