[management, client, proxy] add expose NetBird-only services over tunnel peers (#6226)

Adds a new "private" service mode for the reverse proxy: services reachable exclusively over the embedded WireGuard tunnel, gated by per-peer group membership instead of operator auth schemes.

Wire contract
- ProxyMapping.private (field 13): the proxy MUST call ValidateTunnelPeer and fail closed; operator schemes are bypassed.
- ProxyCapabilities.private (4) + supports_private_service (5): capability gate. Management never streams private mappings to proxies that don't claim the capability; the broadcast path applies the same filter via filterMappingsForProxy.
- ValidateTunnelPeer RPC: resolves an inbound tunnel IP to a peer, checks the peer's groups against service.AccessGroups, and mints a session JWT on success. checkPeerGroupAccess fails closed when a private service has empty AccessGroups.
- ValidateSession/ValidateTunnelPeer responses now carry peer_group_ids + peer_group_names so the proxy can authorise policy-aware middlewares without an extra management round-trip.
- ProxyInboundListener + SendStatusUpdate.inbound_listener: per-account inbound listener state surfaced to dashboards.
- PathTargetOptions.direct_upstream (11): bypass the embedded NetBird client and dial the target via the proxy host's network stack for upstreams reachable without WireGuard.

Data model
- Service.Private (bool) + Service.AccessGroups ([]string, JSON- serialised). Validate() rejects bearer auth on private services. Copy() deep-copies AccessGroups. pgx getServices loads the columns.
- DomainConfig.Private threaded into the proxy auth middleware. Request handler routes private services through forwardWithTunnelPeer and returns 403 on validation failure.
- Account-level SynthesizePrivateServiceZones (synthetic DNS) and injectPrivateServicePolicies (synthetic ACL) gate on len(svc.AccessGroups) > 0.

Proxy
- /netbird proxy --private (embedded mode) flag; Config.Private in proxy/lifecycle.go.
- Per-account inbound listener (proxy/inbound.go) binding HTTP/HTTPS on the embedded NetBird client's WireGuard tunnel netstack.
- proxy/internal/auth/tunnel_cache: ValidateTunnelPeer response cache with single-flight de-duplication and per-account eviction.
- Local peerstore short-circuit: when the inbound IP isn't in the account roster, deny fast without an RPC.
- proxy/server.go reports SupportsPrivateService=true and redacts the full ProxyMapping JSON from info logs (auth_token + header-auth hashed values now only at debug level).

Identity forwarding
- ValidateSessionJWT returns user_id, email, method, groups, group_names. sessionkey.Claims carries Email + Groups + GroupNames so the proxy can stamp identity onto upstream requests without an extra management round-trip on every cookie-bearing request.
- CapturedData carries userEmail / userGroups / userGroupNames; the proxy stamps X-NetBird-User and X-NetBird-Groups on r.Out from the authenticated identity (strips client-supplied values first to prevent spoofing).
- AccessLog.UserGroups: access-log enrichment captures the user's group memberships at write time so the dashboard can render group context without reverse-resolving stale memberships.

OpenAPI/dashboard surface
- ReverseProxyService gains private + access_groups; ReverseProxyCluster gains private + supports_private. ReverseProxyTarget target_type enum gains "cluster". ServiceTargetOptions gains direct_upstream. ProxyAccessLog gains user_groups.

proxy/cmd/proxy/cmd/root.go

@@ -63,6 +63,7 @@ var (
 	preSharedKey          string
 	supportsCustomPorts   bool
 	requireSubdomain      bool
+	private               bool
 	geoDataDir            string
 	crowdsecAPIURL        string
 	crowdsecAPIKey        string
@@ -105,6 +106,8 @@ func init() {
 	rootCmd.Flags().StringVar(&preSharedKey, "preshared-key", envStringOrDefault("NB_PROXY_PRESHARED_KEY", ""), "Define a pre-shared key for the tunnel between proxy and peers")
 	rootCmd.Flags().BoolVar(&supportsCustomPorts, "supports-custom-ports", envBoolOrDefault("NB_PROXY_SUPPORTS_CUSTOM_PORTS", true), "Whether the proxy can bind arbitrary ports for UDP/TCP passthrough")
 	rootCmd.Flags().BoolVar(&requireSubdomain, "require-subdomain", envBoolOrDefault("NB_PROXY_REQUIRE_SUBDOMAIN", false), "Require a subdomain label in front of the cluster domain")
+	rootCmd.Flags().BoolVar(&private, "private", envBoolOrDefault("NB_PROXY_PRIVATE", false), "Enable private services accessible with NetBird-Only authentication mode.")
+	_ = rootCmd.Flags().MarkHidden("private")
 	rootCmd.Flags().DurationVar(&maxDialTimeout, "max-dial-timeout", envDurationOrDefault("NB_PROXY_MAX_DIAL_TIMEOUT", 0), "Cap per-service backend dial timeout (0 = no cap)")
 	rootCmd.Flags().DurationVar(&maxSessionIdleTimeout, "max-session-idle-timeout", envDurationOrDefault("NB_PROXY_MAX_SESSION_IDLE_TIMEOUT", 0), "Cap per-service session idle timeout (0 = no cap)")
 	rootCmd.Flags().StringVar(&geoDataDir, "geo-data-dir", envStringOrDefault("NB_PROXY_GEO_DATA_DIR", "/var/lib/netbird/geolocation"), "Directory for the GeoLite2 MMDB file (auto-downloaded if missing)")
@@ -161,7 +164,8 @@ func runServer(cmd *cobra.Command, args []string) error {
 		return fmt.Errorf("invalid --trusted-proxies: %w", err)
 	}
 
-	srv := proxy.Server{
+	srv := proxy.New(proxy.Config{
+		ListenAddr:               addr,
 		Logger:                   logger,
 		Version:                  Version,
 		ManagementAddress:        mgmtAddr,
@@ -178,7 +182,7 @@ func runServer(cmd *cobra.Command, args []string) error {
 		ACMEChallengeType:        acmeChallengeType,
 		DebugEndpointEnabled:     debugEndpoint,
 		DebugEndpointAddress:     debugEndpointAddr,
-		HealthAddress:            healthAddr,
+		HealthAddr:               healthAddr,
 		ForwardedProto:           forwardedProto,
 		TrustedProxies:           parsedTrustedProxies,
 		CertLockMethod:           nbacme.CertLockMethod(certLockMethod),
@@ -188,12 +192,13 @@ func runServer(cmd *cobra.Command, args []string) error {
 		PreSharedKey:             preSharedKey,
 		SupportsCustomPorts:      supportsCustomPorts,
 		RequireSubdomain:         requireSubdomain,
+		Private:                  private,
 		MaxDialTimeout:           maxDialTimeout,
 		MaxSessionIdleTimeout:    maxSessionIdleTimeout,
 		GeoDataDir:               geoDataDir,
 		CrowdSecAPIURL:           crowdsecAPIURL,
 		CrowdSecAPIKey:           crowdsecAPIKey,
-	}
+	})
 
 	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGTERM, syscall.SIGINT)
 	defer stop()