fix(ci): rebalance backend test splits with measured durations + raise unit-test step timeout (#13583)

Unit Tests - Python 3.12 - Group 3 has been failing at 98-99% on the
nightly: the job hits the nick-fields/retry 40-minute per-attempt timeout
and pytest is SIGTERM'd mid-test, so it looks like one flaky last test
when it is actually a deterministic timeout (and the internal retry plus
run-level retries can never succeed).

Root cause is twofold:

1. .test_durations was last regenerated ~May 2025 and covered only 2,219
   of ~9,500 current unit tests, so pytest-split weighted 77% of the
   suite at the 0.73s average. The expensive client-fixture tests
   (test_webhook.py, test_login.py - each pays a full create_app +
   lifespan boot per test, 60-120s late in a CI run) clustered into
   group 3's tail, making it ~8-10 minutes slower than its siblings
   (33:15 on 3.13, >40min on 3.12 which is additionally slowed by
   astrapy disabling SSL connection reuse on Python 3.12.0-11).
   The weekly store_pytest_durations workflow that should refresh the
   file has been dying at the 6-hour GitHub job limit every week (serial
   full-suite run no longer fits), so the file silently froze.

   This regenerates the file from real per-test wall-clock measured in
   the passing Python 3.13 nightly jobs (run 27246228762 attempt 1, all
   5 groups, parsed from the -vv xdist logs: per-worker start-to-start
   deltas). 9,508 tests now have measured durations; old entries are
   kept where no new measurement exists. Simulated least_duration
   split goes from one outlier group to 5 even groups (~24.6 min each)
   with the >50s tests spread 1-2 per group instead of 7 in one.

2. timeout_minutes 40 -> 50 gives headroom for matrix-cell variance
   (3.12 runs ~20% slower than 3.13) so a slightly slow cell degrades
   gracefully instead of burning 2x40min and failing the whole run.

Follow-ups (not in this PR): fix store_pytest_durations to run with
xdist or split groups so it fits the 6h limit; investigate the in-worker
degradation that makes client-fixture boots cost 8-12s early in a run
but 60-120s after ~30 minutes.

.github/workflows/python_test.yml

@@ -93,7 +93,7 @@ jobs:
       - name: Run unit tests
         uses: nick-fields/retry@v3
         with:
-          timeout_minutes: 40
+          timeout_minutes: 50
           max_attempts: 2
           command: make unit_tests args="-x -vv --splits ${{ matrix.splitCount }} --group ${{ matrix.group }} --reruns 5 --cov --cov-config=src/backend/.coveragerc --cov-report=xml --cov-report=html"