feat: add validation for minimum length on various fields and update type definitions

backend/app/core/time.py

@@ -0,0 +1,11 @@
+from __future__ import annotations
+
+from datetime import UTC, datetime
+
+
+def utcnow() -> datetime:
+    """Return a naive UTC datetime without using deprecated datetime.utcnow()."""
+
+    # Keep naive UTC values for compatibility with existing DB schema/queries.
+    return datetime.now(UTC).replace(tzinfo=None)
+