core: initial Go module skeleton (config, db pool, tenant registry migration)

internal/config/config.go

@@ -0,0 +1,29 @@
+package config
+
+import (
+	"fmt"
+	"os"
+)
+
+// Config holds startup configuration for the core module.
+// RegistryDSN points at the control-plane registry DB (tenant list,
+// connection info, superadmin accounts) — see nexarch-state.json
+// multi_tenancy: Modell C (physisch getrennte DB pro Mandant).
+type Config struct {
+	ListenAddr  string
+	RegistryDSN string
+}
+
+func Load() (Config, error) {
+	dsn := os.Getenv("NEXARCH_REGISTRY_DSN")
+	if dsn == "" {
+		return Config{}, fmt.Errorf("NEXARCH_REGISTRY_DSN not set")
+	}
+
+	addr := os.Getenv("NEXARCH_LISTEN_ADDR")
+	if addr == "" {
+		addr = ":8080"
+	}
+
+	return Config{ListenAddr: addr, RegistryDSN: dsn}, nil
+}

internal/db/db.go

@@ -0,0 +1,11 @@
+package db
+
+import (
+	"context"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+func Connect(ctx context.Context, dsn string) (*pgxpool.Pool, error) {
+	return pgxpool.New(ctx, dsn)
+}