Health Check

Liveness/health check for Railway monitoring and the in-container watchdog.

Declared async (not def) so it is served directly on the event loop and never needs an anyio threadpool slot. A flood of slow synchronous endpoints (e.g. /v1/categories’ 3-4s aggregation) can pin all 40 threadpool threads; a sync /health would queue behind them, miss the watchdog’s consecutive liveness checks, and trip the hard-exit crash loop (issue #203). Returning a static payload off the threadpool keeps liveness answerable under that load.