Fix/datetime utcnow deprecation

[Acuspeedster]

Closes #139

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Summary

Replaces deprecated datetime.utcnow() with timezone-aware datetime.now(timezone.utc) in ORM models.

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Problem

• datetime.utcnow() deprecated in Python 3.12
• Produces naive datetime objects
• Can cause comparison errors
• Not future-proof

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Solution

• Introduced private helper:

def _utcnow():
    return datetime.now(timezone.utc)

• Used _utcnow as default_factory
• Imported timezone from datetime

Using a named helper keeps default_factory serializable and explicit.

Impact

• Fully timezone-aware timestamps
• Forward-compatible with Python 3.12+
• No schema migration required