Add Config struct and NewHandlerWithConfig(logger, cfg, checks...) constructor.
Config.CheckTimeout sets the per-request deadline for all health checks; zero
value defaults to 5 seconds. NewHandler remains unchanged as a backward-compatible
wrapper. API committed as stable.
HTTP health check handler with parallel goroutine-per-check execution, 5 s request-derived timeout, and two-level criticality (LevelCritical → 503, LevelDegraded → 200).
What's included:
- `Checkable` interface (HealthCheck / Name / Priority) and `Level` type with LevelCritical and LevelDegraded constants
- `NewHandler(logger, checks...)` returning http.Handler; runs all checks concurrently via buffered channel, returns JSON with per-component status and latency
- `ComponentStatus` and `Response` types for the JSON response body
Tested-via: todo-api POC integration
Reviewed-against: docs/adr/
