Fix regression auth restart CI

[wayyoungboy]

Summary

• run regression CI API server with a single worker to avoid multi-worker startup contention during auth-mode restarts
• dump server.log when regression tests fail so future startup failures expose the server-side error

Verification

• source /data/code/wayyoung/powermem/.venv/bin/activate
• locally verified make server-start -> health OK with auth disabled, then auth enabled restart -> health OK
• git diff --check
• parsed .github/workflows/regression.yml with PyYAML

[wayyoungboy]

Closing this PR per maintainer request.