chore: migrate pre-commit + lint stack to ruff

[cmyui]

Summary

Replaces five pre-commit hooks (black, pyupgrade, reorder-python-imports, add-trailing-comma, blacken-docs) with ruff-check + ruff-format. Two orders of magnitude faster, single config file, no more in-fighting hooks.

Config

New `pyproject.toml` (project's first) with ruff config that keeps the existing style:

• `F` / `I` / `UP` / `COM` rulesets cover what the old hooks did.
• `COM812` disabled (conflicts with formatter).
• `F401` disabled for now — codebase has ~16 real unused imports that predate ruff. Cleaning them up deserves its own PR rather than landing as churn here.
• isort: `force-single-line = true` + `required-imports = ["from future import annotations"]` to mirror reorder-python-imports.
• `docstring-code-format = true` replaces blacken-docs.

What ruff autofix changed

Small, mechanical: 2× `UP017` (`datetime.timezone.utc` → `datetime.UTC`), 1× `F541` (redundant `f""`), 4× `I001` (minor import reorderings). Net ~15 lines across 10 files.

Follow-ups I'd do next

• Enable `F401` + remove the ~16 genuinely unused imports.
• Rename `app/models/_init.py` (three underscores, typo) to `init.py`. Currently a dead barrel file; `app.models` works as an implicit namespace package by accident.
• Drop `from future import annotations` entirely — the `required-imports` rule is the single place to remove it from now, and on Python 3.11+ it's cosmetic.

Test plan

• `make utest` — 31 pass
• `make itest` — 7 pass
• `mypy .` clean
• `pre-commit run --all-files` clean (two hooks instead of nine)

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