chore: remove unused UniqueConstraint import from database/models.py

[devin-ai-integration]

chore: remove unused UniqueConstraint import from database/models.py

Summary

Removes the unused UniqueConstraint import from the SQLAlchemy import block in database/models.py. The symbol was speculatively imported alongside Index during initial code generation but is never referenced — only Index is actually used in FXRate.__table_args__.

Review & Testing Checklist for Human

• Verify UniqueConstraint is not referenced anywhere else in the codebase (a repo-wide grep for UniqueConstraint should return zero results after this change)

Notes

• Candidate ID: 9b29743be31b
• Devin Session
• Requested by: bot_apk

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring