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/store/postgres/postgres.go

@@ -0,0 +1,27 @@
+package postgres
+
+import (
+	"context"
+	"database/sql"
+	_ "github.com/jackc/pgx/v5/stdlib"
+)
+
+type DB struct {
+	sql *sql.DB
+}
+
+func Open(url string) (*DB, error) {
+	db, err := sql.Open("pgx", url)
+	if err != nil {
+		return nil, err
+	}
+	return &DB{sql: db}, nil
+}
+
+func (d *DB) Close() error {
+	return d.sql.Close()
+}
+
+func (d *DB) Ping(ctx context.Context) error {
+	return d.sql.PingContext(ctx)
+}