Add ruff and rumdl to enforce style on this repo

[gaurav]

Nothing enforces style here today: no ruff config, no rumdl config, and .github/workflows/tests.yml runs pytest only. This is to add both and wire them into CI.

It already pays for itself

Running ruff for the first time found a real latent bug, not just style:

tests/test_db_download.py:68:30: F821 Undefined name `Path`
tests/test_db_download.py:73:38: F821 Undefined name `Path`
tests/test_db_download.py:79:37: F821 Undefined name `Path`

_FakeEnsure annotates three signatures with Path and never imports it. The tests pass because from __future__ import annotations makes annotations strings that are never evaluated — so this is invisible until something evaluates them (typing.get_type_hints, a runtime validator, or simply dropping the future import). That is exactly the class of error nobody catches by reading.

Already fixed in bc2b76d, confirmed real rather than cosmetic: before the fix typing.get_type_hints(_FakeEnsure.ensure) raised NameError: name 'Path' is not defined; after it the three hints resolve to pathlib.Path. Counts as evidence for this issue, not against it — the linter found it on its first run, before any config existed.

Current state, measured

Tool	Finding
ruff check .	24 errors, 8 auto-fixable (was 27; bc2b76d fixed all three F821s with one import)
ruff format --diff .	15 files would be reformatted
rumdl check .	56 issues in 5 files, 11 auto-fixable
lines > 88 chars	101, across src/ and tests/

Top ruff rules by count in what remains: PLW1510 subprocess-run-without-check (5), ISC004 implicit-string-concatenation (4), BLE001 blind-except (3), I001 unsorted-imports (3).

Decisions this needs before it is a mechanical change

• Line length. 101 lines exceed ruff's default 88. The code here is deliberately comment-heavy and several of those are prose in docstrings, so the choice is a longer limit (100?) or reflowing a lot of explanation. Picking 88 and auto-formatting would rewrap comments that were laid out by hand.
• rumdl's MD013 at 80 columns. This repo's Markdown is hard-wrapped at ~80 already, but tables, code blocks and long links legitimately exceed it — 56 issues is mostly this. Either raise the limit or exempt tables/code.
• Which ruff rules. The 27 above are ruff's defaults plus what it surfaces with a broad selection; BLE001 and PLW1510 in particular are judgement calls in test code, where a bare except or an unchecked subprocess.run is often deliberate.
• ruff format or not. Adopting the formatter is a one-time large diff that will bury git blame for every file. Worth doing, but worth doing as its own commit so it can be added to .git-blame-ignore-revs.

Suggested order

1. Land a pyproject.toml config for both with the decisions above.
2. One commit of pure auto-fixes (ruff check --fix, ruff format, rumdl fmt), separate and blame-ignorable.
3. One commit fixing what is left by hand — including the F821 above, which should arguably be fixed on its own before any of this.
4. Add a lint job to .github/workflows/tests.yml, last, so CI is green the moment it appears.

Doing (4) first would put a red X on every PR until the rest lands.