Add health package: REST and gRPC health checks

- Handler(db) returns http.HandlerFunc: 200 ok / 503 unhealthy
- RegisterGRPC registers grpc.health.v1.Health on a gRPC server
- 4 tests: healthy, unhealthy (closed db), content type, gRPC registration

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

health/health.go

@@ -0,0 +1,48 @@
+// Package health provides standard health check implementations for
+// Metacircular services, supporting both REST and gRPC.
+package health
+
+import (
+	"database/sql"
+	"encoding/json"
+	"net/http"
+
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/health"
+	healthpb "google.golang.org/grpc/health/grpc_health_v1"
+)
+
+// Handler returns an http.HandlerFunc that checks database connectivity.
+// It returns 200 {"status":"ok"} if the database is reachable, or
+// 503 {"status":"unhealthy","error":"..."} if the ping fails.
+//
+// Mount it on whatever path the service uses (typically /healthz or
+// /v1/health).
+func Handler(database *sql.DB) http.HandlerFunc {
+	return func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+
+		if err := database.Ping(); err != nil {
+			w.WriteHeader(http.StatusServiceUnavailable)
+			_ = json.NewEncoder(w).Encode(map[string]string{
+				"status": "unhealthy",
+				"error":  err.Error(),
+			})
+			return
+		}
+
+		w.WriteHeader(http.StatusOK)
+		_ = json.NewEncoder(w).Encode(map[string]string{
+			"status": "ok",
+		})
+	}
+}
+
+// RegisterGRPC registers the standard gRPC health checking service
+// (grpc.health.v1.Health) on the given gRPC server. The health server
+// is set to SERVING status immediately.
+func RegisterGRPC(srv *grpc.Server) {
+	hs := health.NewServer()
+	hs.SetServingStatus("", healthpb.HealthCheckResponse_SERVING)
+	healthpb.RegisterHealthServer(srv, hs)
+}