add ci

[tsk-brc]

Note

Low Risk
Mostly CI/lint configuration plus broad formatting/import cleanup across the Django app; functional behavior should be unchanged aside from stricter lint enforcement via .flake8.

Overview
Adds a GitHub Actions CI workflow (.github/workflows/ci.yml) to run linting and pytest+coverage on pushes/PRs, and centralizes flake8 settings in a new .flake8 file; CircleCI/Makefile lint commands are simplified to rely on that config.

Updates dev dependencies (requirements-dev.txt) and applies wide-ranging formatting/import cleanup across shift/ and shift_table/ (admin, models, views, factories, and tests), largely standardizing quoting and line wrapping without intended logic changes.

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

There are 3 total unresolved issues (including 2 from previous reviews).

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