feat(health): initial stable release v0.9.0
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/
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