Add cancelTask to SchedulerService for cancelling running tasks

Adds the ability to cancel currently running scheduled tasks via a new
cancelTask method on the SchedulerService interface. For global (distributed)
tasks, the database lock is released and a periodic liveness check detects
the lost ticket and aborts the task function's AbortSignal. For local tasks,
the abort signal is triggered directly. Also adds a REST endpoint at
POST /.backstage/scheduler/v1/tasks/:id/cancel.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Fredrik Adelöw <freben@spotify.com>

packages/backend-plugin-api/src/services/definitions/SchedulerService.ts

@@ -304,6 +304,16 @@ export interface SchedulerService {
    */
   triggerTask(id: string): Promise<void>;
 
+  /**
+   * Cancels a currently running task by ID, marking it as idle.
+   *
+   * If the task doesn't exist, a NotFoundError is thrown. If the task is
+   * not currently running, a ConflictError is thrown.
+   *
+   * @param id - The task ID
+   */
+  cancelTask(id: string): Promise<void>;
+
   /**
    * Schedules a task function for recurring runs.
    *