refactor: split backchecks.py into a subpackage (#197)

[iabaako]

Pull Request Summary 🚀

What does this PR do? 📝

Splits src/datasure/checks/backchecks.py (4,252 lines) into a subpackage, src/datasure/checks/backchecks/:

• models.py — Pydantic settings/result models, StrEnums, and module constants
• compute.py — pure compute_*/_calculate_*/_add_*_columns data-processing functions (no Streamlit calls)
• settings_ui.py — backchecks_report_settings and its config-UI helpers
• report_ui.py — backchecks_report and its report-rendering UI helpers (including the backcheck-column-configuration UI, which turned out to be called only from the report entry point, not settings)
• __init__.py — left empty, matching every other package in this repo

Also eliminates two intra-file duplicates found while splitting:

• _render_no_differences_settings/_render_exclude_values_settings shared an identical add/remove-popover list-editor pattern → extracted _render_value_list_editor, with a guard_remove_membership flag preserving the one subtle behavioral difference between the two callers
• _add_extra_survey_columns/_add_extra_backcheck_columns shared a select→rename→suffix→join pattern → extracted _join_extra_columns

Tests are mirrored into tests/checks/backchecks/{conftest,test_models,test_compute,test_settings_ui,test_report_ui}.py, replacing the 4,303-line tests/checks/test_backchecks.py.

This is the second of four planned splits tracked by #197 (enumerator done in #265, gpschecks/outliers still pending), stacked on top of refactor/197-split-enumerator-module since that PR is still open.

Why is this change needed? 🤔

Per #197: backchecks.py was the largest and hardest-to-review check module. It mixed settings models, pure computation, and two distinct Streamlit UI flows (settings vs. report) in one 4,252-line file.

How was this implemented? 🛠️

Pure mechanical reorganization — no logic changes, aside from the two documented deduplications above (both verified to produce identical output via the existing test suite). Grepped the full src/ tree to confirm only output_view_template.py imports from this module externally, and updated it to import backchecks_report directly from datasure.checks.backchecks.report_ui instead of relying on package re-exports. The generated, gitignored output_view_1.py was left untouched, per this repo's convention of never editing generated output views directly — it's regenerated from the template.

How to test or reproduce ? 🧪

1. uv run python -m pytest tests/checks/backchecks --cov=datasure.checks.backchecks --cov-report=term-missing -q — 220/220 tests pass (exact match to the pre-split count), coverage improved from 89.95% to 91.34%
2. uv run python -m pytest -q -m "not slow" — full suite passes, same pre-existing skips
3. Confirm the Backchecks report page in the app still renders, and its settings still save/load correctly

Screenshots (if applicable) 📷

N/A — no UI or behavior changes.

Checklist ✅

• I have run and tested my changes locally
• I have limit this PR to less than 1000 lines of code change (if not, explain why) — ~5,900 lines changed, but this is a pure file-move/reorganization plus two small, verified dedup extractions; splitting it into multiple PRs would fragment a single mechanical refactor for no review benefit
• I have updated/added tests to cover my changes (if applicable)
• I have updated/added requirements to cover my changes (if applicable)
• I have run linting and formatting on any code changes (if applicable)
• I have updated the documentation (README, etc.) accordingly
• I have reviewed and resolved any merge conflict

🤖 Generated with Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
7 New issues
0 Accepted issues

Measures
0 Security Hotspots
91.3% Coverage on New Code
1.1% Duplication on New Code

See analysis details on SonarQube Cloud