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/server.ts

@@ -1058,7 +1058,7 @@ export function createServer(store: TaskStore, options?: ServerOptions): ReturnT
   app.get("/api/health", (_req, res) => {
     const database = store.getDatabaseHealth();
     res.json({
-      status: database.corruptionDetected ? "degraded" : "ok",
+      status: database.healthy ? "ok" : "degraded",
       version: cliPackageVersion,
       uptime: Math.floor(process.uptime()),
       database,