Fix CI lint failures in backend db module

[reloadfast]

Motivation

• CI was failing due to lint errors in backend/app/db.py: an undefined logger, import-order issues, overly long f-string log messages, and missing trailing newline.

Description

• Add a module-level logger with logger = logging.getLogger(__name__) and move the logging import to satisfy import-order checks.
• Replace long f-string logger.error(...) calls with parameterized, multiline logger.error(...) calls to fix undefined-name and line-length lint errors.
• Reformat the file to satisfy ruff and ensure the file ends with a trailing newline.

Testing

• Ran cd backend && ruff check . which passed.
• Ran cd backend && ruff format --check . which passed.
• Ran frontend checks: npm run lint, npx prettier --check "src/**/*.{ts,tsx,css}", and npm run test:coverage, all of which passed.
• cd backend && pytest -m "unit or integration" --cov=app --cov-report=term-missing --cov-fail-under=80 -q could not be completed in the local environment due to Python 3.10 missing tomllib and datetime.UTC (CI runs on Python 3.11 where these tests are expected to collect normally).

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Codex Task