chore: replace deprecated datetime.utcnow Column defaults with datetime.now(timezone.utc) in database/models.py

[devin-ai-integration]

chore: replace deprecated datetime.utcnow Column defaults with datetime.now(timezone.utc)

Summary

Replaces the deprecated default=datetime.utcnow callable in FXRate.scraped_at and MidMarketRate.scraped_at Column definitions with default=lambda: datetime.now(timezone.utc).replace(tzinfo=None).

datetime.utcnow was deprecated in Python 3.12 (this repo runs on 3.12.8) and emits DeprecationWarning on every row INSERT since SQLAlchemy invokes the default callable each time. The replacement uses .replace(tzinfo=None) to continue producing naive UTC datetimes, maintaining compatibility with the existing SQLite schema and any code that reads these columns.

Review & Testing Checklist for Human

• Verify that .replace(tzinfo=None) is the right approach for this codebase — if other parts of the app expect timezone-aware datetimes, this would need to change
• Note that other datetime.utcnow() call sites exist in main.py and scrapers/base.py and are not addressed here — confirm this scoping is intentional
• Recommended test: run python main.py --demo and inspect the scraped_at values in fx_fees.db to confirm they are still naive UTC datetimes

Notes

• Candidate ID: 9f56079177a1
• Devin Session
• Requested by: bot_apk

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