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>

tests/worker/agents/session-cleanup-helper.test.ts

@@ -37,6 +37,7 @@ describe('SessionCleanupHelper', () => {
       earliestPendingTimestamp: Date.now() - 10000, // 10 seconds ago
       conversationHistory: [],
       currentProvider: 'claude',
+      processingMessageIds: [],  // CLAIM-CONFIRM pattern: track message IDs being processed
       ...overrides,
     };
   }