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>

internal/db/models.go

@@ -316,4 +316,5 @@ type WeekPlan struct {
 	CalendarSyncedAt sql.NullString
 	CreatedAt        string
 	UpdatedAt        string
+	Revision         int64
 }