fix: persist session completion to database in completeByDbId (#1532)

completeByDbId only cleaned up in-memory state, leaving sdk_sessions rows
with status='active' and completed_at=NULL indefinitely. Ghost sessions
accumulated and exhausted the agent pool, causing 60s timeout errors.

- Add SessionStore.markSessionCompleted() to set status/completed_at/completed_at_epoch
- Call it at the start of completeByDbId before in-memory cleanup
- Inject SessionStore into SessionCompletionHandler via constructor
- Add 4 tests covering status, timestamps, isolation, and non-existent IDs

Closes #1532

Co-Authored-By: Claude <noreply@anthropic.com>

src/services/worker/session/SessionCompletionHandler.ts

@@ -11,12 +11,14 @@
 
 import { SessionManager } from '../SessionManager.js';
 import { SessionEventBroadcaster } from '../events/SessionEventBroadcaster.js';
+import { SessionStore } from '../../sqlite/SessionStore.js';
 import { logger } from '../../../utils/logger.js';
 
 export class SessionCompletionHandler {
   constructor(
     private sessionManager: SessionManager,
-    private eventBroadcaster: SessionEventBroadcaster
+    private eventBroadcaster: SessionEventBroadcaster,
+    private sessionStore: SessionStore
   ) {}
 
   /**
@@ -24,6 +26,9 @@ export class SessionCompletionHandler {
    * Used by DELETE /api/sessions/:id and POST /api/sessions/:id/complete
    */
   async completeByDbId(sessionDbId: number): Promise<void> {
+    // Persist completion to database before in-memory cleanup (fix for #1532)
+    this.sessionStore.markSessionCompleted(sessionDbId);
+
     // Delete from session manager (aborts SDK agent)
     await this.sessionManager.deleteSession(sessionDbId);