[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/internal/tcp/router_test.go

@@ -1739,3 +1739,97 @@ func TestCheckRestrictions_IPv4MappedIPv6(t *testing.T) {
 	connOutside := &fakeConn{remote: fakeAddr("[::ffff:192.168.1.1]:5678")}
 	assert.NotEqual(t, restrict.Allow, router.checkRestrictions(connOutside, route), "::ffff:192.168.1.1 not in v4 CIDR")
 }
+
+// TestRouter_PlainHTTP_RoutesToPlainChannel verifies that a plain (non-TLS)
+// connection lands on the plain HTTP channel when the router was built
+// with WithPlainHTTP, leaving the TLS channel untouched.
+func TestRouter_PlainHTTP_RoutesToPlainChannel(t *testing.T) {
+	logger := log.StandardLogger()
+	addr := &net.TCPAddr{IP: net.IPv4(127, 0, 0, 1), Port: 443}
+
+	router := NewRouter(logger, nil, addr, WithPlainHTTP(addr))
+	router.AddRoute("example.com", Route{Type: RouteHTTP})
+
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	require.NoError(t, err, "test listener bind must succeed")
+	defer ln.Close()
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	go func() {
+		_ = router.Serve(ctx, ln)
+	}()
+
+	// Plain HTTP request (no TLS handshake byte).
+	go func() {
+		conn, err := net.DialTimeout("tcp", ln.Addr().String(), 2*time.Second)
+		if err != nil {
+			return
+		}
+		_, _ = conn.Write([]byte("GET / HTTP/1.1\r\nHost: example.com\r\n\r\n"))
+	}()
+
+	plainListener := router.HTTPListenerPlain()
+	require.NotNil(t, plainListener, "plain listener must be exposed when WithPlainHTTP is set")
+
+	acceptDone := make(chan net.Conn, 1)
+	go func() {
+		conn, err := plainListener.Accept()
+		if err == nil {
+			acceptDone <- conn
+		}
+	}()
+
+	select {
+	case conn := <-acceptDone:
+		require.NotNil(t, conn)
+		_ = conn.Close()
+	case <-router.HTTPListener().(*chanListener).ch:
+		t.Fatal("plain HTTP request leaked into TLS channel")
+	case <-time.After(3 * time.Second):
+		t.Fatal("plain HTTP connection never reached plain channel")
+	}
+}
+
+// TestRouter_TLS_StaysOnTLSChannel_WhenPlainEnabled verifies that the
+// presence of a plain channel does not divert TLS traffic — TLS still
+// goes to the TLS channel as before.
+func TestRouter_TLS_StaysOnTLSChannel_WhenPlainEnabled(t *testing.T) {
+	logger := log.StandardLogger()
+	addr := &net.TCPAddr{IP: net.IPv4(127, 0, 0, 1), Port: 443}
+
+	router := NewRouter(logger, nil, addr, WithPlainHTTP(addr))
+	router.AddRoute("example.com", Route{Type: RouteHTTP})
+
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	require.NoError(t, err, "test listener bind must succeed")
+	defer ln.Close()
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	go func() { _ = router.Serve(ctx, ln) }()
+
+	// Send a TLS ClientHello.
+	go func() {
+		conn, err := net.DialTimeout("tcp", ln.Addr().String(), 2*time.Second)
+		if err != nil {
+			return
+		}
+		tlsConn := tls.Client(conn, &tls.Config{
+			ServerName:         "example.com",
+			InsecureSkipVerify: true, //nolint:gosec
+		})
+		_ = tlsConn.Handshake()
+		_ = tlsConn.Close()
+	}()
+
+	select {
+	case conn := <-router.httpCh:
+		require.NotNil(t, conn, "TLS conn should land on the TLS channel")
+		_ = conn.Close()
+	case <-time.After(5 * time.Second):
+		t.Fatal("TLS conn never reached the TLS channel")
+	}
+}