chore: add ruff + mypy + CI tooling (and fix a pyarrow NameError)

[dylanuys]

Sets up linting, type-checking, and CI — and fixes the real bug the linter immediately surfaced.

Real bug found by linting

download.py caught except (OSError, pyarrow.ArrowInvalid) in 5 places, but only import pyarrow.parquet as pq was present — pyarrow was never bound. So whenever a corrupt parquet raised ArrowInvalid, the handler itself raised NameError instead of catching it. Fixed by importing pyarrow.

Tooling

• ruff ([tool.ruff]): baseline ruleset E4,E7,E9,F — pyflakes (real bugs) plus import/statement errors. Style rules left off for now to keep the diff focused. Blocking in CI.
• mypy ([tool.mypy]): lenient config (ignore_missing_imports). Advisory in CI (continue-on-error) — the codebase is ~30% typed, so it reports without blocking until coverage improves.
• dev extra: pip install -e .[dev] now pulls pytest/ruff/mypy (previously no test dep was declared anywhere).
• CI (.github/workflows/ci.yml): lint (blocking) + typecheck (advisory) on push/PR to dev/main.

Lint fixes applied to reach a clean baseline

• Real bug: pyarrow import (above).
• Bare except: → except Exception: (4 sites in archive.py/media.py — they were swallowing KeyboardInterrupt/SystemExit).
• Removed unused imports (16), empty f-strings (27), redundant local imports, and dead local variables (incl. 3 ignored df = await run_*_benchmark(...) captures).
• Two x = lambda → def in download.py.

All mechanical; no behavior change except the pyarrow fix and the bare-except narrowing.

Verification

• ruff check src tests → clean.
• Package imports; pyarrow.ArrowInvalid now resolves.
• Unit suite unchanged: 39 pass, same 5 pre-existing test_config_loading failures.

Deliberately out of scope (next step)

No test job in CI yet, for two reasons that need your call:

1. The 5 failing test_config_loading tests are stale/brittle — m4a and mov are valid formats (they're in download.py's extension sets), and the count/membership tests hardcode values that drift on every data change. Fixing them means deciding what they should assert.
2. A test job pulls the full heavy dep set (torch, transformers, onnxruntime, …) — a CI time/cost decision.

Happy to follow up: fix the stale tests (derive format validity from the real extension constants; make the count tests robust) and add a blocking test job.