feat: Introduce SessionEventBroadcaster and SessionCompletionHandler for improved session management

- Added SessionEventBroadcaster to handle broadcasting of session lifecycle events, consolidating SSE broadcasting and processing status updates.
- Refactored SessionRoutes to utilize SessionEventBroadcaster for broadcasting events related to new prompts, session starts, and completions.
- Created SessionCompletionHandler to centralize session completion logic, reducing duplication across multiple endpoints.
- Updated WorkerService to initialize SessionEventBroadcaster and pass it to SessionRoutes.

src/services/worker/events/SessionEventBroadcaster.ts

@@ -0,0 +1,96 @@
+/**
+ * Session Event Broadcaster
+ *
+ * Provides semantic broadcast methods for session lifecycle events.
+ * Consolidates SSE broadcasting and processing status updates.
+ */
+
+import { SSEBroadcaster } from '../SSEBroadcaster.js';
+import type { WorkerService } from '../../worker-service.js';
+
+export class SessionEventBroadcaster {
+  constructor(
+    private sseBroadcaster: SSEBroadcaster,
+    private workerService: WorkerService
+  ) {}
+
+  /**
+   * Broadcast new user prompt arrival
+   * Starts activity indicator to show work is beginning
+   */
+  broadcastNewPrompt(prompt: {
+    id: number;
+    claude_session_id: string;
+    project: string;
+    prompt_number: number;
+    prompt_text: string;
+    created_at_epoch: number;
+  }): void {
+    // Broadcast prompt details
+    this.sseBroadcaster.broadcast({
+      type: 'new_prompt',
+      prompt
+    });
+
+    // Start activity indicator (work is about to begin)
+    this.sseBroadcaster.broadcast({
+      type: 'processing_status',
+      isProcessing: true
+    });
+
+    // Update processing status based on queue depth
+    this.workerService.broadcastProcessingStatus();
+  }
+
+  /**
+   * Broadcast session initialization
+   */
+  broadcastSessionStarted(sessionDbId: number, project: string): void {
+    this.sseBroadcaster.broadcast({
+      type: 'session_started',
+      sessionDbId,
+      project
+    });
+
+    // Update processing status
+    this.workerService.broadcastProcessingStatus();
+  }
+
+  /**
+   * Broadcast observation queued
+   * Updates processing status to reflect new queue depth
+   */
+  broadcastObservationQueued(sessionDbId: number): void {
+    this.sseBroadcaster.broadcast({
+      type: 'observation_queued',
+      sessionDbId
+    });
+
+    // Update processing status (queue depth changed)
+    this.workerService.broadcastProcessingStatus();
+  }
+
+  /**
+   * Broadcast session completion
+   * Updates processing status to reflect session removal
+   */
+  broadcastSessionCompleted(sessionDbId: number): void {
+    this.sseBroadcaster.broadcast({
+      type: 'session_completed',
+      timestamp: Date.now(),
+      sessionDbId
+    });
+
+    // Update processing status (session removed from queue)
+    this.workerService.broadcastProcessingStatus();
+  }
+
+  /**
+   * Broadcast summarize request queued
+   * Updates processing status to reflect new queue depth
+   */
+  broadcastSummarizeQueued(): void {
+    // Update processing status (queue depth changed)
+    this.workerService.broadcastProcessingStatus();
+  }
+}