chore: add pre-commit hooks (ruff)

[ayoub3bidi]

Adds .pre-commit-config.yaml with ruff check --fix and ruff-format matching requirements.txt v0.8.4.

Made with Cursor

[ayoub3bidi]

Branch cleanup

Reset to main and cherry-picked only the pre-commit commit (.pre-commit-config.yaml only — 1 file).

Verification

Check	Result
docker compose run mercury_linter (ruff check, CI-equivalent)	All checks passed
pre-commit run --all-files (venv)	ruff passed; ruff-format would reformat 10 files on first run (format-only; not included in this PR)
Fresh Docker API stack	Flyway OK, logs clean
GET /v1/health	200
POST /v1/token	200

Config pins ruff-pre-commit at v0.8.4, matching requirements.txt.

Ready to merge.