feat(FN-4029): expose database health in API and store contracts

Adds database health endpoints to the API layer (FN-4029), exposing a new store health accessor through `api-node.ts` and `api/legacy.ts`, with aligned auth middleware integration tests and updated architecture documentation.

Fusion-Task-Id: FN-4029

packages/dashboard/src/__tests__/auth-middleware-integration.test.ts

@@ -66,9 +66,9 @@ class MockStore extends EventEmitter {
 
   getDatabaseHealth() {
     return {
-      corruptionDetected: false,
-      integrityCheckPending: false,
-      integrityCheckLastRunAt: null,
+      healthy: true,
+      isRunning: false,
+      lastCheckedAt: null,
     };
   }