Keep one draft in flight per week

A confirmed draft asked for while the week's draft job was asked for
without confirm no longer enqueues a second job. With two job workers the
two drafts ran at once, and whichever stored second failed plan_changed,
so the household's confirmed draft could fail. StartDraft now refuses it
with ErrDraftRunning, 409 draft_running, and enqueues nothing; the
household asks again once the running draft finishes. Every other draft
joins the week's queued or running job, so a week has at most one.

The plan tests run the job runner with two workers, as cmd/server does,
and a confirmed draft during a running unconfirmed one is refused while
only the running job drafts.

Maestro task ea48e650-d510-49cc-8485-0f07174ee731.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/ARCHITECTURE.md

@@ -345,8 +345,9 @@ automatic** (D-021).
   crock-pot dinner with known minutes, else null. `POST
   /api/v1/plans/{weekStart}/draft` takes `{confirm?}` and answers 202
   `{jobId}` for a `draft_week` job, or for the one already queued or
-  running for the week — a confirmed one when the draft is confirmed, so a
-  confirmed draft never joins an unconfirmed job. The job runs the `draft_week` skill on every Regular
+  running for the week, so a week has at most one draft in flight. A
+  confirmed draft while the week's job was asked for without confirm is
+  409 `draft_running` and enqueues nothing. The job runs the `draft_week` skill on every Regular
   and every Quest log dinner no other plan for the current or a later week
   has, with the household context and last week's dinners, and stores the
   week: a new plan is `draft`; a redraft keeps each day's