[P1/bug] Deprecated datetime.utcnow() — 104 occurrences across 43 files (breaks on Python 3.14)

[yeongseon]

Description

datetime.utcnow() has been deprecated since Python 3.12 and is removed in Python 3.14. The cookbook uses it in 104 places across 43 files. Some files already correctly use datetime.now(timezone.utc) — this should be made consistent.

Affected Areas

• fundamentals/pycubrid/07_merge_upsert.py, 09_serial_order_numbers.py
• templates/api-service-fastapi/recipes/ — 12+ files, heaviest offender
• templates/flask/ — 15+ files
• Various other recipes

Impact

• DeprecationWarning on Python 3.12+
• AttributeError on Python 3.14 (method removed)

Files already correct (reference pattern)

• fundamentals/pycubrid/11_bulk_etl_pipeline.py — uses datetime.now(timezone.utc)
• fundamentals/pycubrid/14_manual_cascade_delete.py — uses datetime.now(timezone.utc)

Fix

Replace all occurrences:

# Before
datetime.utcnow()

# After (timezone-aware)
datetime.now(timezone.utc)

# After (naive UTC, if timezone-aware causes issues)
datetime.now(timezone.utc).replace(tzinfo=None)

Add from datetime import timezone to imports where missing.

Context

Found during line-by-line code review (2025-07-23).

[yeongseon]

Fixed in PR #34 (merged).