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/http/routes/SessionRoutes.ts

@@ -40,7 +40,8 @@ export class SessionRoutes extends BaseRouteHandler {
     super();
     this.completionHandler = new SessionCompletionHandler(
       sessionManager,
-      eventBroadcaster
+      eventBroadcaster,
+      dbManager.getSessionStore()
     );
   }