feat: scaffold agentmon services and k8s deploy

Adds Go microservices (ingest-gateway, event-processor, query-api, web-ui), NATS+Postgres wiring, initial schema/init job, ingress manifests for LAN+tailnet, and a multi-arch image build script.

internal/httpx/json.go

@@ -0,0 +1,12 @@
+package httpx
+
+import (
+	"encoding/json"
+	"net/http"
+)
+
+func WriteJSON(w http.ResponseWriter, status int, v any) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	_ = json.NewEncoder(w).Encode(v)
+}