Guard plan drafts against swaps and keep confirm on single-flight

Return the draft job id as jobId, as the plan API contract specifies.

Add a week_plans revision (migration 0013) that every swap and stored
draft raises. A draft_week job reads the revision before draft_week runs
and stores its week only while the plan still has it; a plan edited
while the AI ran is left as it is and the job fails plan_changed, so a
draft never erases the household's swap.

A confirmed draft now joins only a confirmed queued or running job, so a
job asked for without confirm can no longer fail it with
confirm_required after the plan was synced.

Test simultaneous drafts, a swap during a draft, a confirmed draft while
an unconfirmed one runs, impossible calendar dates and start_by across
the end of daylight saving time.

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

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

migrations/0013_week_plan_revision.sql

@@ -0,0 +1,6 @@
+-- 0013_week_plan_revision: every edit of a week's plan — a swap or a
+-- stored draft — raises its revision. A draft_week job reads the revision
+-- before draft_week runs and stores its week only while the plan still
+-- has it, so a draft never overwrites an edit made while the AI ran.
+
+ALTER TABLE week_plans ADD COLUMN revision INTEGER NOT NULL DEFAULT 0;