feat: Implement Worker Service v2 with improved architecture

- Complete rewrite of the Worker Service following object-oriented principles.
- Introduced a single long-lived database connection to reduce overhead.
- Implemented event-driven queues to eliminate polling.
- Added DRY utilities for pagination and settings management.
- Reduced code size significantly from 1173 lines to approximately 600-700 lines.
- Created various composed services including DatabaseManager, SessionManager, and SDKAgent.
- Enhanced SSE broadcasting capabilities for real-time client updates.
- Established a structured approach for session lifecycle management and event handling.
- Introduced type safety with shared TypeScript interfaces for better maintainability.

src/services/worker/SSEBroadcaster.ts

@@ -0,0 +1,83 @@
+/**
+ * SSEBroadcaster: SSE client management
+ *
+ * Responsibility:
+ * - Manage SSE client connections
+ * - Broadcast events to all connected clients
+ * - Handle disconnections gracefully
+ * - Single-pass broadcast (no two-step cleanup)
+ */
+
+import type { Response } from 'express';
+import { logger } from '../../utils/logger.js';
+import type { SSEEvent, SSEClient } from '../worker-types.js';
+
+export class SSEBroadcaster {
+  private sseClients: Set<SSEClient> = new Set();
+
+  /**
+   * Add a new SSE client connection
+   */
+  addClient(res: Response): void {
+    this.sseClients.add(res);
+    logger.debug('WORKER', 'Client connected', { total: this.sseClients.size });
+
+    // Setup cleanup on disconnect
+    res.on('close', () => {
+      this.removeClient(res);
+    });
+
+    // Send initial event
+    this.sendToClient(res, { type: 'connected', timestamp: Date.now() });
+  }
+
+  /**
+   * Remove a client connection
+   */
+  removeClient(res: Response): void {
+    this.sseClients.delete(res);
+    logger.debug('WORKER', 'Client disconnected', { total: this.sseClients.size });
+  }
+
+  /**
+   * Broadcast an event to all connected clients (single-pass)
+   */
+  broadcast(event: SSEEvent): void {
+    if (this.sseClients.size === 0) {
+      return; // Short-circuit if no clients
+    }
+
+    const eventWithTimestamp = { ...event, timestamp: Date.now() };
+    const data = `data: ${JSON.stringify(eventWithTimestamp)}\n\n`;
+
+    // Single-pass write + cleanup
+    for (const client of this.sseClients) {
+      try {
+        client.write(data);
+      } catch (err) {
+        // Remove failed client immediately
+        this.sseClients.delete(client);
+        logger.debug('WORKER', 'Client removed due to write error');
+      }
+    }
+  }
+
+  /**
+   * Get number of connected clients
+   */
+  getClientCount(): number {
+    return this.sseClients.size;
+  }
+
+  /**
+   * Send event to a specific client
+   */
+  private sendToClient(res: Response, event: SSEEvent): void {
+    const data = `data: ${JSON.stringify(event)}\n\n`;
+    try {
+      res.write(data);
+    } catch (err) {
+      this.sseClients.delete(res);
+    }
+  }
+}