fix: worker startup crash and missing observation columns

Two bugs fixed:

1. SessionCompletionHandler called dbManager.getSessionStore() during
   WorkerService construction, before DB initialization. Changed to
   accept DatabaseManager and defer the call to runtime.

2. migration009 (generated_by_model, relevance_count columns) only ran
   via the deprecated MigrationRunner path, never through SessionStore's
   migration chain. Added addObservationModelColumns() to SessionStore
   constructor. Checks column existence directly since schema_versions
   may have been marked applied without the ALTER TABLE succeeding.

Also removed duplicate transcriptWatcher declaration and shutdown block
(merge artifact).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/services/worker/session/SessionCompletionHandler.ts

@@ -11,14 +11,14 @@
 
 import { SessionManager } from '../SessionManager.js';
 import { SessionEventBroadcaster } from '../events/SessionEventBroadcaster.js';
-import { SessionStore } from '../../sqlite/SessionStore.js';
+import { DatabaseManager } from '../DatabaseManager.js';
 import { logger } from '../../../utils/logger.js';
 
 export class SessionCompletionHandler {
   constructor(
     private sessionManager: SessionManager,
     private eventBroadcaster: SessionEventBroadcaster,
-    private sessionStore: SessionStore
+    private dbManager: DatabaseManager
   ) {}
 
   /**
@@ -27,7 +27,7 @@ export class SessionCompletionHandler {
    */
   async completeByDbId(sessionDbId: number): Promise<void> {
     // Persist completion to database before in-memory cleanup (fix for #1532)
-    this.sessionStore.markSessionCompleted(sessionDbId);
+    this.dbManager.getSessionStore().markSessionCompleted(sessionDbId);
 
     // Delete from session manager (aborts SDK agent via SIGTERM)
     await this.sessionManager.deleteSession(sessionDbId);