feat(backend): B19 paired t-test

[az-said]

Summary

• Adds POST /api/cohorts/compare running Welch's two-sample t-test on a chosen biochemical metric between two precomputed cohorts.
• New backend/services/cohort_stats.py with welch_t_test(a, b) → {t, p, df} — thin wrapper around scipy.stats.ttest_ind(equal_var=False). Cleans None / NaN, requires n >= 2 per group, falls back to a manual Welch-Satterthwaite df calc on scipy < 1.11.
• New backend/api/routes/cohorts.py registered in api/main.py. Reads precomputed JSON from backend/data/precomputed/<id>.json (M2 artifact). Validates: metric is a Literal{muH, hydrophobicity, charge}, a != b, alphanumeric+._- dataset ids only (path-traversal guard), missing precomputed file → 404 with a helpful "run M2" message, n < 2 → 422 (not 500).
• Adds scipy >= 1.14, < 2.0 to backend/requirements.txt. ~50MB image bump, but unlocks every downstream stats endpoint (rank tests, distribution comparisons) the cohort and ranking workflows will want.

Test plan

• `pytest backend/tests/test_cohort_stats.py -v` — 11 cases all green
  • 4 unit: scipy parity, NaN/None drop, n<2 rejection, identical-distribution sanity
  • 7 route: happy path, NULL skipping, missing dataset, unsupported metric, identical cohorts, path traversal, too-few-samples
• Manual: build a stub precomputed file, then
  `curl -X POST http://localhost:8000/api/cohorts/compare -H "Content-Type: application/json" -d '{"a":"peleg_118","b":"uperin_frog","metric":"muH"}'`

Dependency

The route reads from backend/data/precomputed/<id>.json produced by M2. The route returns a clean 404 when the artifact is missing, so the merge order between B19 and M2 is flexible.

Out of scope (per dispatch)

• No changes to backend/schemas/api_models.py response shapes.
• No changes to process_upload_dataframe internals.

Closes B19 from `docs/internal/WAVE_2_8_DISPATCH_2026_06_18.md` §3.

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📥 Commits

Reviewing files that changed from the base of the PR and between 834d90a and 76a6386.

📒 Files selected for processing (5)

• backend/api/main.py
• backend/api/routes/cohorts.py
• backend/requirements.txt
• backend/services/cohort_stats.py
• backend/tests/test_cohort_stats.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/backend-b19-welch-ttest

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