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/migrations/0007_customuser_signature.py

@@ -0,0 +1,16 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('users', '0006_customuser_work_position'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='customuser',
+            name='signature',
+            field=models.TextField(blank=True, default='', verbose_name='Email potpis'),
+        ),
+    ]

backend/core/users/migrations/0008_prefill_empty_signatures.py

@@ -0,0 +1,39 @@
+from django.db import migrations
+
+
+def prefill_empty_signatures(apps, schema_editor):
+    CustomUser = apps.get_model('users', 'CustomUser')
+    for user in CustomUser.objects.all().iterator():
+        signature_value = user.signature if isinstance(user.signature, str) else ''
+        if signature_value.strip():
+            continue
+        first_name = (user.first_name or '').strip()
+        last_name = (user.last_name or '').strip()
+        full_name = f"{first_name} {last_name}".strip()
+        fallback_name = (user.email or '').strip() or 'Ime i prezime'
+        display_name = full_name or fallback_name
+        occupation = (user.occupation or '').strip()
+        name_line = f"{occupation} {display_name}".strip() if occupation else display_name
+        mobile = (user.telefon or '').strip() or "+385 91 636 3466"
+        signature = (
+            "Poslano iz aplikacije\n"
+            "S poštovanjem / Freundliche Grüße / Kind regards\n\n"
+            f"{name_line}\n\n"
+            "KNEZ LJUBO d.o.o.\n"
+            "Ivlje 44, 10040 Zagreb\n\n"
+            "Office: Donje Svetice 40, 10000 Zagreb\n\n"
+            f"Mobile: {mobile}\n\n"
+            "www.knezljubo.hr"
+        )
+        user.signature = signature
+        user.save(update_fields=['signature'])
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ('users', '0007_customuser_signature'),
+    ]
+
+    operations = [
+        migrations.RunPython(prefill_empty_signatures, migrations.RunPython.noop),
+    ]