fix: use UTC import instead of datetime.UTC attribute

[GeiserX]

Summary

• from datetime import datetime makes datetime the class, not the module
• datetime.UTC only exists on the module level, causing AttributeError: type object 'datetime.datetime' has no attribute 'UTC' at runtime
• Fixed by importing UTC directly: from datetime import UTC, datetime, timedelta

Discovered during v0.2.71 production deployment — collection runs were failing with this error.

Files changed

• app/database.py — 6 occurrences
• app/main.py — 2 occurrences

Test plan

• ruff check passes
• pytest tests/test_auth.py — 15/15 passing
• python3 -c "from datetime import UTC, datetime; datetime.now(UTC)" works

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 12c7b72c-25b4-4971-a470-689fda1e77fe

📥 Commits

Reviewing files that changed from the base of the PR and between 000659e and 84933c2.

📒 Files selected for processing (2)

• app/database.py
• app/main.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/datetime-utc-import

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