Use datetime.UTC for Python 3.13+ compatibility

[Copilot]

Addresses review feedback on PR #36 regarding deprecated datetime usage in app/common/http_response/base.py.

Changes

• Replace timezone.utc with datetime.UTC in BaseResponse timestamp field
• Remove unused timezone import

The datetime.UTC constant is the modern Python 3.11+ approach and is recommended for projects using Python 3.13+.

# Before
from datetime import datetime, timezone
timestamp: datetime = Field(default_factory=lambda: datetime.now(timezone.utc))

# After  
from datetime import datetime
timestamp: datetime = Field(default_factory=lambda: datetime.now(datetime.UTC))

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