docs: add POST /api/rebuild-queue/{id}/cancel endpoint (follow-up to #469)

docs/web-ui.md

@@ -447,6 +447,11 @@ not ours.
   the sentinel `[cancelled]`. Same code path as a real answer.
 - `POST /request-spawn` — queue a Spawn approval.
 - `POST /update-all` — rebuild every stale container.
+- `POST /api/rebuild-queue/{id}/cancel` — drop a `Queued` entry
+  (#447). Refuses `Running` / terminal-state entries (in-flight
+  rebuilds can't be safely interrupted). Always 200; body is
+  `{"cancelled": true}` on a successful flip or
+  `{"cancelled": false}` when the entry was not in `Queued` state.
 - `POST /op-send` (`to=<name>`, `body=<text>`) — drop an
   operator-authored message into `<name>`'s inbox. `to=*` fans
   out to every registered agent. Returns 200; the broker