MAESTRO: fix(db): prevent FK constraint failures on worker restart

Cherry-picked source changes from PR #889 by @Et9797. Fixes #846.

Key changes:
- Add ensureMemorySessionIdRegistered() guard in SessionStore.ts
- Add ON UPDATE CASCADE migration (schema v21) for observations and session_summaries FK constraints
- Change message queue from claim-and-delete to claim-confirm pattern (PendingMessageStore.ts)
- Add spawn deduplication and unrecoverable error detection in SessionRoutes.ts and worker-service.ts
- Add forceInit flag to SDKAgent for stale session recovery

Build artifacts skipped (pre-existing dompurify dep issue). Path fixes (HealthMonitor.ts, worker-utils.ts)
already merged via PR #634.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/services/worker/SessionManager.ts

@@ -154,7 +154,8 @@ export class SessionManager {
       earliestPendingTimestamp: null,
       conversationHistory: [],  // Initialize empty - will be populated by agents
       currentProvider: null,  // Will be set when generator starts
-      consecutiveRestarts: 0  // Track consecutive restart attempts to prevent infinite loops
+      consecutiveRestarts: 0,  // Track consecutive restart attempts to prevent infinite loops
+      processingMessageIds: []  // CLAIM-CONFIRM: Track message IDs for confirmProcessed()
     };
 
     logger.debug('SESSION', 'Creating new session object (memorySessionId cleared to prevent stale resume)', {