feat(task): add task labels and push-based task events

ts/taskbuffer.interfaces.ts

@@ -17,6 +17,7 @@ export interface ITaskMetadata {
   timeout?: number;
   lastError?: string;
   errorCount?: number;
+  labels?: Record<string, string>;
 }
 
 export interface ITaskExecutionReport {
@@ -38,4 +39,14 @@ export interface IScheduledTaskInfo {
   lastRun?: Date;
   steps?: ITaskStep[];
   metadata?: ITaskMetadata;
+}
+
+export type TTaskEventType = 'started' | 'step' | 'completed' | 'failed';
+
+export interface ITaskEvent {
+  type: TTaskEventType;
+  task: ITaskMetadata;
+  timestamp: number;
+  stepName?: string;   // present when type === 'step'
+  error?: string;      // present when type === 'failed'
 }