Investigate and report (no code changes)

task_id: invest07
task_number: 7
title: "Diagnose intermittent 500s on POST /api/orders since deploy abc123"
prompt: |
  Since commit `abc123` we see intermittent 500s
  on POST /api/orders, roughly 1 in 30 requests.
  The Sentry trace points at
  `services/orders.ts:142`. Reproduce locally if
  you can, identify the root cause, and write
  your findings to `INVESTIGATION.md` at the
  repo root.

  This is a diagnosis task, not a fix task. Do
  NOT change any code outside of
  `INVESTIGATION.md`. The follow-up fix will be
  a separate task once we agree on the proposed
  approach.

  `INVESTIGATION.md` must include sections:
  - _Symptom_ — what users see.
  - _Reproduction_ — exact commands that
    reproduce locally (or "could not reproduce"
    with what you tried).
  - _Root cause_ — the actual mechanism, with
    file:line references.
  - _Proposed fix_ — the smallest plausible
    change, as a bulleted spec for a future task.
  - _Open questions_ — anything you couldn't
    resolve without product input.
acceptance_criteria: |
  - `INVESTIGATION.md` exists at the repo root
    with the five sections above.
  - The Root cause section names at least one
    specific `file:line` location.
  - No code changes outside `INVESTIGATION.md`
    (verify with `git diff --stat`).
  - The file is valid Markdown (renders cleanly
    in GitHub preview).
  - `npm run lint` passes (no incidental
    formatting changes).
status: ready