Push the week's plan to Google Calendar (Feature 16)

POST /api/v1/plans/{weekStart}/push-calendar writes one event per plan
day to the chosen calendar: "Dinner: {name}" from the household dinner
time for 60 minutes in the household zone, with the dinner name,
"Start by {time}" for crock-pot dinners and the
{MEALQUEST_PUBLIC_URL}/dinner/{id} link. Event ids are the lowercase
base32hex SHA-256 of "mealquest|{weekStart}|{date}"; each event is
inserted, and a 409 updates it instead.

gcal.Service.PushEvents does the Calendar writes under the account lock,
with the same redaction as listing. plan.Service.Push records a push only
when every event was written: the plan becomes synced with
calendar_synced_at and each day stores its event id. A partial failure
records nothing; a plan edited mid-push is 409 plan_changed.

A swap, or a redraft to another dinner, clears the day's
calendar_event_id, so the next push writes just those days; a push with
none pending writes every day again.

No calendar chosen is 409 no_calendar; a deleted or read-only chosen
calendar is 409 calendar_unavailable; a refused grant is 502
reconnect_required, leaving the connection as it is.

Maestro task 5885d2b1-cf06-44ff-abf3-242d7763c3c9.

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

internal/plan/draft.go

@@ -136,9 +136,10 @@ func (s *Service) request(ctx context.Context, weekStart store.LocalDate) (skill
 
 // storeDraft stores week as the plan of the week starting weekStart in one
 // transaction and returns the plan's id. A new plan is draft. A draft
-// replaces the days of an existing plan, keeping each day's calendar
-// event id; a draft plan stays draft and a synced or changed one becomes
-// changed, since Calendar no longer shows it. A plan synced since the
+// replaces the days of an existing plan, keeping the calendar event id of
+// each day whose dinner stays the same and clearing the others'; a draft
+// plan stays draft and a synced or changed one becomes changed, since
+// Calendar no longer shows it. A plan synced since the
 // draft was asked for without confirm is left as it is, failing with
 // ReasonConfirmRequired, and so is a plan no longer at revision base (the
 // revision before draft_week ran, or noPlan), failing with