ci: add Ruff lint check

[tigdav]

Summary

• Added ruff (pinned) to requirements-dev.txt.
• Added backend/pyproject.toml with a [tool.ruff] configuration: line length 120, target Python 3.11, auto-generated migrations excluded, and a curated lint rule set (E, F, W, B, UP, C4, SIM).
• Added a Ruff lint step to the CI workflow that runs ruff check . before the tests.

Notes

• Linting only — no application code was reformatted or modified. The current codebase already passes ruff check . cleanly under this configuration.
• The rule set was chosen so it passes on the existing code without changes; import sorting (I) and stricter line length were intentionally left out to avoid touching source in this PR.
• Verified locally with the pinned ruff version: ruff check . exits 0.