Let an on-call day be handed to somebody else

A date is held by exactly one person and POST /api/schedule plain-inserts,
so any date that was already taken came back 409. That made reassignment
impossible through the API: the only route was to delete the entry first,
and for a week that meant seven separate deletions. Worse, the reject is
all-or-nothing across the request, so assigning a week where a single day
happened to be taken failed entirely and placed none of the other six.

The refusal itself is worth keeping. Moving a shift off the person
expecting to be paged for it should not be something a plain call does by
accident, so the fix is to make it possible to ask for rather than to
remove the guard: "replace": true takes the dates anyway, and the flag
defaults to off so every existing caller behaves exactly as before.

The delete and the insert share the transaction that was already there.
That matters more than the flag does — a week of free and taken days now
lands as a unit, and a failure part way through leaves the rota as it was
instead of with a shift deleted and nothing put back. A rota with a hole
in it is worse than a rota that refused to change.

One consequence worth naming: under replace a date repeated inside one
request is idempotent rather than a conflict, because the second pass
clears what the first wrote.

README.md

@@ -190,6 +190,13 @@ A new incident is assigned to whoever holds today's schedule entry at the moment
 it opens (`GET /api/schedule/current`). If nobody is scheduled it opens
 unassigned. Reassign with `POST /api/incidents/{id}/assign`.
 
+One person holds a given day, so `POST /api/schedule` refuses a date somebody
+already has: taking a shift off the person expecting to be paged for it should
+not be something a plain call does by accident. Pass `"replace": true` to take
+them anyway. Either way the whole request is one transaction — a week where some
+days are free and some are taken moves as a unit, and a failure leaves the rota
+exactly as it was rather than with a hole in it.
+
 ### Push notifications
 
 With `TERDUT_NTFY_URL` set, an incident that opens is pushed to the on-call
@@ -467,7 +474,7 @@ unknown" rather than being rejected.
 
 | Method | Path | Description |
 |---|---|---|
-| `POST` | `/api/schedule` | Assign user to dates `{"user_id", "dates":["YYYY-MM-DD",...]}` — all-or-nothing |
+| `POST` | `/api/schedule` | Assign user to dates `{"user_id", "dates":["YYYY-MM-DD",...], "replace"}` — all-or-nothing |
 | `GET` | `/api/schedule` | List entries. Filters: `?from=YYYY-MM-DD`, `?to=YYYY-MM-DD` |
 | `GET` | `/api/schedule/current` | Today's on-call user (UTC), 404 if none |
 | `DELETE` | `/api/schedule/{id}` | Remove schedule entry |