feat: add ticket domain models

backend/apps/tickets/tests/__init__.py

@@ -0,0 +1 @@
+

backend/apps/tickets/tests/test_ticket_status.py

@@ -0,0 +1,12 @@
+import pytest
+
+from apps.tickets.services import TicketStatusError, validate_status_transition
+
+
+def test_pending_ticket_can_move_to_in_progress():
+    validate_status_transition("pending", "in_progress")
+
+
+def test_resolved_ticket_cannot_move_back_to_in_progress():
+    with pytest.raises(TicketStatusError, match="resolved -> in_progress"):
+        validate_status_transition("resolved", "in_progress")