fix: replace deprecated datetime.utcnow in database models

[Arijit429]

Problem

datetime.utcnow is deprecated since Python 3.12 and was missed
in the previous fix (PR #449). It is used as default_factory
in both Template and FormSubmission models in api/db/models.py.

Fix

• Import timezone from datetime
• Replace default_factory=datetime.utcnow with
  default_factory=lambda: datetime.now(timezone.utc) in both models

Files Changed

• api/db/models.py

Closes #[your issue number]