feat(server): support cancellation of running generation jobs (#2190)

examples/server/api.md

@@ -1037,7 +1037,11 @@ Notes:
 
 #### `POST /sdcpp/v1/jobs/{id}/cancel`
 
-Attempts to cancel an accepted job.
+Cancels a queued job immediately, or requests interruption of a generating image or
+video job. A generating job returns `200 OK` with `cancel_requested: true` and
+remains `generating` until execution stops at a cancellation checkpoint. Continue
+polling until its status becomes `cancelled`. Cancelled jobs have no result.
+Repeated cancellation requests are safe; terminal jobs are returned unchanged.
 
 Typical status codes: