Initial scaffold with per-nameserver DNS monitoring model

Full project structure following upaas conventions: uber/fx DI, go-chi
routing, slog logging, Viper config. State persisted as JSON file with
per-nameserver record tracking for inconsistency detection. Stub
implementations for resolver, portcheck, tlscheck, and watcher.

internal/handlers/healthcheck.go

@@ -0,0 +1,17 @@
+package handlers
+
+import (
+	"net/http"
+)
+
+// HandleHealthCheck returns the health check handler.
+func (h *Handlers) HandleHealthCheck() http.HandlerFunc {
+	return func(
+		writer http.ResponseWriter,
+		request *http.Request,
+	) {
+		h.respondJSON(
+			writer, request, h.hc.Check(), http.StatusOK,
+		)
+	}
+}