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-service.ts

@@ -199,9 +199,6 @@ export class WorkerService {
   // Stale session reaper interval (Issue #1168)
   private staleSessionReaperInterval: ReturnType<typeof setInterval> | null = null;
 
-  // Transcript watcher for external CLI sessions (e.g. Codex, Gemini)
-  private transcriptWatcher: TranscriptWatcher | null = null;
-
   // AI interaction tracking for health endpoint
   private lastAiInteraction: {
     timestamp: number;
@@ -992,13 +989,6 @@ export class WorkerService {
       this.staleSessionReaperInterval = null;
     }
 
-    // Stop transcript watcher
-    if (this.transcriptWatcher) {
-      this.transcriptWatcher.stop();
-      this.transcriptWatcher = null;
-      logger.info('SYSTEM', 'Transcript watcher stopped');
-    }
-
     await performGracefulShutdown({
       server: this.server.getHttpServer(),
       sessionManager: this.sessionManager,