[BUG] Deprecated datetime.utcnow() causes warnings on Python 3.12+

[Arijit429]

Problem

datetime.utcnow() is used in api/db/models.py and api/routes/templates.py.
This is deprecated in Python 3.12+ and produces warnings visible in pytest output:

[Arijit429]

Fix

Replace with datetime.now(datetime.UTC) in both files.

[Arijit429]

Working on a fix in PR #449.