ci: Ruff lint errors blocking PR merges

[anchapin]

Issue

Multiple Ruff lint errors (F401 - unused imports) are failing CI checks:

Files with unused imports fixed in PRs #1328, #1329:

• ai_engine/mmsd/premium_client.py - json, field, httpx
• ai_engine/tests/test_premium_client.py - ConversionResult, MagicMock, httpx

Total pre-existing errors found across codebase:

```
Found 6373 errors.
[*] 4276 fixable with the --fix option
```

Fix

1. Run ruff check --fix . to auto-fix fixable issues
2. Manually fix remaining issues requiring judgment
3. Add a pre-commit hook to catch these early
4. Consider adding ruff check to CI with fail-under threshold