fix : replace deprecated datetime.utcnow with timezone-aware alternative in time_tracking

[tmdeveloper007]

Summary of What Needs to be Done

Replace usage of the deprecated datetime.utcnow() in src/utils/time_tracking.py with the timezone-aware datetime.now(timezone.utc).

Changes that Need to be Made

In src/utils/time_tracking.py:

1. Add timezone to the import: from datetime import datetime, timedelta, timezone
2. Replace all occurrences of datetime.utcnow() with datetime.now(timezone.utc) in start_session, calculate_learning_velocity, estimate_completion_time, and predict_skill_proficiency methods

Impact that it Would Provide

• Fixes Python 3.12+ deprecation warning for datetime.utcnow()
• Uses the recommended timezone-aware datetime approach
• Correct timestamp generation for time tracking records

Note: Please assign this issue to the tmdeveloper007 account.