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/tests/test_email_signature.py

@@ -0,0 +1,39 @@
+import uuid
+
+import pytest
+from django.contrib.auth import get_user_model
+
+
+@pytest.mark.django_db
+def test_new_user_gets_prefilled_signature():
+    User = get_user_model()
+    suffix = uuid.uuid4().hex[:8]
+    user = User.objects.create_user(
+        email=f"signature-{suffix}@example.test",
+        password="test1234",
+        first_name="Mario",
+        last_name="Tkalac",
+        telefon="+385916363466",
+    )
+
+    signature = user.signature
+    assert signature.startswith("Poslano iz aplikacije\n")
+    assert "S poštovanjem / Freundliche Grüße / Kind regards" in signature
+    assert "Mario Tkalac" in signature
+    assert "KNEZ LJUBO d.o.o." in signature
+    assert "Office: Donje Svetice 40, 10000 Zagreb" in signature
+    assert "Mobile: +385916363466" in signature
+    assert "www.knezljubo.hr" in signature
+
+
+@pytest.mark.django_db
+def test_custom_signature_is_preserved():
+    User = get_user_model()
+    user = User.objects.create_user(
+        email=f"custom-signature-{uuid.uuid4().hex[:8]}@example.test",
+        password="test1234",
+        signature="Custom email signature",
+    )
+
+    assert user.signature == "Custom email signature"
+    assert user.get_email_signature() == "Custom email signature"