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/email_utils.py

@@ -0,0 +1,17 @@
+def append_user_signature(body, user):
+    body_text = str(body or '').strip()
+    if user is None:
+        return body_text
+
+    signature = ''
+    get_signature = getattr(user, 'get_email_signature', None)
+    if callable(get_signature):
+        signature = str(get_signature() or '').strip()
+    else:
+        signature = str(getattr(user, 'signature', '') or '').strip()
+
+    if not signature:
+        return body_text
+    if not body_text:
+        return signature
+    return f"{body_text}\n\n{signature}"