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/response-processor.test.ts

@@ -72,6 +72,8 @@ describe('ResponseProcessor', () => {
     mockDbManager = {
       getSessionStore: () => ({
         storeObservations: mockStoreObservations,
+        ensureMemorySessionIdRegistered: mock(() => {}),  // FK fix (Issue #846)
+        getSessionById: mock(() => ({ memory_session_id: 'memory-session-456' })),  // FK fix (Issue #846)
       }),
       getChromaSync: () => ({
         syncObservation: mockChromaSyncObservation,
@@ -85,6 +87,7 @@ describe('ResponseProcessor', () => {
       },
       getPendingMessageStore: () => ({
         markProcessed: mock(() => {}),
+        confirmProcessed: mock(() => {}),  // CLAIM-CONFIRM pattern: confirm after successful storage
         cleanupProcessed: mock(() => 0),
         resetStuckMessages: mock(() => 0),
       }),
@@ -126,6 +129,7 @@ describe('ResponseProcessor', () => {
       earliestPendingTimestamp: Date.now() - 10000,
       conversationHistory: [],
       currentProvider: 'claude',
+      processingMessageIds: [],  // CLAIM-CONFIRM pattern: track message IDs being processed
       ...overrides,
     };
   }
@@ -269,6 +273,8 @@ describe('ResponseProcessor', () => {
       }));
       (mockDbManager.getSessionStore as any) = () => ({
         storeObservations: mockStoreObservations,
+        ensureMemorySessionIdRegistered: mock(() => {}),
+        getSessionById: mock(() => ({ memory_session_id: 'memory-session-456' })),
       });
 
       await processAgentResponse(
@@ -367,6 +373,8 @@ describe('ResponseProcessor', () => {
       }));
       (mockDbManager.getSessionStore as any) = () => ({
         storeObservations: mockStoreObservations,
+        ensureMemorySessionIdRegistered: mock(() => {}),
+        getSessionById: mock(() => ({ memory_session_id: 'memory-session-456' })),
       });
 
       await processAgentResponse(
@@ -446,6 +454,8 @@ describe('ResponseProcessor', () => {
       }));
       (mockDbManager.getSessionStore as any) = () => ({
         storeObservations: mockStoreObservations,
+        ensureMemorySessionIdRegistered: mock(() => {}),
+        getSessionById: mock(() => ({ memory_session_id: 'memory-session-456' })),
       });
 
       await processAgentResponse(
@@ -477,6 +487,8 @@ describe('ResponseProcessor', () => {
       }));
       (mockDbManager.getSessionStore as any) = () => ({
         storeObservations: mockStoreObservations,
+        ensureMemorySessionIdRegistered: mock(() => {}),
+        getSessionById: mock(() => ({ memory_session_id: 'memory-session-456' })),
       });
 
       await processAgentResponse(
@@ -519,6 +531,8 @@ describe('ResponseProcessor', () => {
       }));
       (mockDbManager.getSessionStore as any) = () => ({
         storeObservations: mockStoreObservations,
+        ensureMemorySessionIdRegistered: mock(() => {}),
+        getSessionById: mock(() => ({ memory_session_id: 'memory-session-456' })),
       });
 
       await processAgentResponse(
@@ -555,6 +569,8 @@ describe('ResponseProcessor', () => {
       }));
       (mockDbManager.getSessionStore as any) = () => ({
         storeObservations: mockStoreObservations,
+        ensureMemorySessionIdRegistered: mock(() => {}),
+        getSessionById: mock(() => ({ memory_session_id: 'memory-session-456' })),
       });
 
       await processAgentResponse(
@@ -595,6 +611,8 @@ describe('ResponseProcessor', () => {
       }));
       (mockDbManager.getSessionStore as any) = () => ({
         storeObservations: mockStoreObservations,
+        ensureMemorySessionIdRegistered: mock(() => {}),
+        getSessionById: mock(() => ({ memory_session_id: 'memory-session-456' })),
       });
 
       await processAgentResponse(
@@ -615,7 +633,7 @@ describe('ResponseProcessor', () => {
   });
 
   describe('error handling', () => {
-    it('should throw error if memorySessionId is missing', async () => {
+    it('should throw error if memorySessionId is missing from session', async () => {
       const session = createMockSession({
         memorySessionId: null, // Missing memory session ID
       });