Add plan service and weekly plan API

Add internal/plan: a draft_week job that drafts the week through the
harness from every Regular and the Quest log dinners no other current or
later plan holds, given the household context and last week's dinners,
and stores it as the week's plan; plan reads with the week label, is_new
and each crock-pot dinner's start_by (dinner time minus minutes_total,
computed on read); and day swaps that recompute is_new and move a synced
plan to changed. Drafting over a synced or changed plan needs confirm, at
enqueue and again when the job stores the week; a redraft keeps each
day's calendar event id.

Serve GET /api/v1/plans/{weekStart}, POST /api/v1/plans/{weekStart}/draft
(202 {job_id}, single-flight per week) and PUT
/api/v1/plans/{weekStart}/days/{date}; a weekStart that is not a Monday
is 400 invalid_week.

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

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

docs/ARCHITECTURE.md

@@ -334,6 +334,31 @@ automatic** (D-021).
   the decision's own transaction, so a decision is never recorded without
   the replenish it calls for; when they cannot be written, neither is the
   decision, and a retry decides again.
+- Weekly plans (`plan.Service`). `{weekStart}` is a household-local Monday
+  (`YYYY-MM-DD`); any other value is 400 `invalid_week`. `GET
+  /api/v1/plans/{weekStart}` returns `{id, week_start, week_label, status,
+  ai_note, calendar_synced_at, updated_at, days}`, or 404 when the week has
+  no plan; `days` are the seven days Monday first, each `{date, dinner: {id,
+  name, photo, minutes, method, origin}, is_new, start_by}`. `is_new` means
+  the dinner was planned from the Quest log; `start_by` is the household's
+  dinner time on that date minus `minutes_total`, computed on read, for a
+  crock-pot dinner with known minutes, else null. `POST
+  /api/v1/plans/{weekStart}/draft` takes `{confirm?}` and answers 202
+  `{job_id}` for a `draft_week` job, or for the one already queued or
+  running for the week. 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
+  `calendar_event_id`, and a `synced` or `changed` plan becomes `changed`.
+  Drafting over a `synced` or `changed` plan needs `{"confirm": true}`, else
+  409 `confirm_required`; a plan synced while an unconfirmed draft runs is
+  left as it is and the job fails `confirm_required`. A pool under seven
+  dinners fails the job `not_enough_dinners`, draft_week failing
+  `ai_failed`. `PUT /api/v1/plans/{weekStart}/days/{date}` takes
+  `{dinner_id}`, a Regular or Quest log dinner, swaps the day, recomputes
+  `is_new`, moves a `synced` plan to `changed`, and answers 200 with the
+  plan. A date outside the week is 400 `invalid_date`, an unknown dinner or
+  a week with no plan 404, and any other dinner 409 `dinner_not_plannable`.
 - `/health` — liveness.
 - Everything else serves the embedded PWA with SPA fallback.