Health check endpoint for Kubernetes liveness/readiness probes.

IMPORTANT: This endpoint must be extremely fast and lightweight.

• No database operations
• No external service calls
• Just returns a static response

async def, deliberately (2026-09-09). As a plain def it ran in the shared thread pool, so during a class transition the probe queued behind 46 threads of dashboard reads and took 25 s — which is how two pods were pulled from the Service on 2026-09-08 while serving fine. A static return cannot block the loop; answering from the loop means the probe waits only for the loop's next turn, never for a thread.