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
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