[management,signal,proxy,relay,misc] Unify service configuration loading

Service entry points currently resolve defaults, files, environment variables, and flags differently, which makes precedence inconsistent and prevents some services from using config files.

Introduce one Viper-backed loader and migrate Combined, Management, Relay, Signal, and Proxy while preserving compatibility aliases and Management template expansion.

management/cmd/config_test.go

@@ -0,0 +1,50 @@
+package cmd
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestLoadManagementConfigSources(t *testing.T) {
+	t.Setenv("MANAGEMENT_DATA_DIR", "/template-data")
+	t.Setenv("MANAGEMENT_ENCRYPTION_KEY", "template-key")
+	t.Setenv("NB_DATADIR", "/environment-data")
+	t.Setenv("NB_HTTPCONFIG_AUTHAUDIENCE", "environment-audience")
+	configPath := filepath.Join(t.TempDir(), "management.json")
+	require.NoError(t, os.WriteFile(configPath, []byte(`{
+  "Datadir": "{{ .MANAGEMENT_DATA_DIR }}",
+  "DataStoreEncryptionKey": "{{ .MANAGEMENT_ENCRYPTION_KEY }}",
+  "HttpConfig": {
+    "AuthAudience": "file-audience"
+  },
+  "TURNConfig": {
+    "CredentialsTTL": "1h"
+  }
+}`), 0o600))
+
+	cfg, err := loadManagementConfig(configPath)
+	require.NoError(t, err)
+	assert.Equal(t, "/environment-data", cfg.Datadir, "Bound environment values should override template values")
+	assert.Equal(t, "template-key", cfg.DataStoreEncryptionKey, "Template environment values should be expanded")
+	require.NotNil(t, cfg.HttpConfig, "Nested configuration should be decoded")
+	assert.Equal(t, "environment-audience", cfg.HttpConfig.AuthAudience, "Bound environment values should override the file")
+	require.NotNil(t, cfg.TURNConfig, "TURN configuration should be decoded")
+	assert.Equal(t, time.Hour, cfg.TURNConfig.CredentialsTTL.Duration, "JSON duration types should be decoded")
+
+	datadirFlag := mgmtCmd.Flags().Lookup("datadir")
+	oldDatadir := datadirFlag.Value.String()
+	oldChanged := datadirFlag.Changed
+	t.Cleanup(func() {
+		require.NoError(t, datadirFlag.Value.Set(oldDatadir))
+		datadirFlag.Changed = oldChanged
+	})
+	require.NoError(t, datadirFlag.Value.Set("/flag-data"))
+	datadirFlag.Changed = true
+	ApplyCommandLineOverrides(cfg, mgmtCmd.Flags())
+	assert.Equal(t, "/flag-data", cfg.Datadir, "Flags should override environment values")
+}