feat(private-service): expose NetBird-only services over tunnel peers

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.

management/internals/shared/grpc/proxy_group_access_test.go

@@ -129,6 +129,14 @@ func (m *mockUsersManager) GetUser(ctx context.Context, userID string) (*types.U
 	return user, nil
 }
 
+func (m *mockUsersManager) GetUserWithGroups(ctx context.Context, userID string) (*types.User, []*types.Group, error) {
+	user, err := m.GetUser(ctx, userID)
+	if err != nil {
+		return nil, nil, err
+	}
+	return user, nil, nil
+}
+
 func TestValidateUserGroupAccess(t *testing.T) {
 	tests := []struct {
 		name             string
@@ -420,3 +428,46 @@ func TestGetAccountProxyByDomain(t *testing.T) {
 		})
 	}
 }
+
+func TestCheckPeerGroupAccess(t *testing.T) {
+	t.Run("private with empty AccessGroups denies", func(t *testing.T) {
+		svc := &service.Service{Private: true, AccessGroups: nil}
+		err := checkPeerGroupAccess(svc, []string{"grp-admins"})
+		require.Error(t, err)
+		assert.Contains(t, err.Error(), "no access groups")
+	})
+
+	t.Run("private with peer in AccessGroups allows", func(t *testing.T) {
+		svc := &service.Service{Private: true, AccessGroups: []string{"grp-admins", "grp-ops"}}
+		assert.NoError(t, checkPeerGroupAccess(svc, []string{"grp-other", "grp-ops"}))
+	})
+
+	t.Run("private with peer outside AccessGroups denies", func(t *testing.T) {
+		svc := &service.Service{Private: true, AccessGroups: []string{"grp-admins"}}
+		assert.Error(t, checkPeerGroupAccess(svc, []string{"grp-other"}))
+	})
+
+	t.Run("bearer enabled with empty DistributionGroups allows", func(t *testing.T) {
+		svc := &service.Service{
+			Auth: service.AuthConfig{BearerAuth: &service.BearerAuthConfig{Enabled: true}},
+		}
+		assert.NoError(t, checkPeerGroupAccess(svc, []string{"grp-anyone"}))
+	})
+
+	t.Run("bearer enabled gates on DistributionGroups", func(t *testing.T) {
+		svc := &service.Service{
+			Auth: service.AuthConfig{
+				BearerAuth: &service.BearerAuthConfig{
+					Enabled:            true,
+					DistributionGroups: []string{"grp-allowed"},
+				},
+			},
+		}
+		assert.NoError(t, checkPeerGroupAccess(svc, []string{"grp-allowed"}))
+		assert.Error(t, checkPeerGroupAccess(svc, []string{"grp-other"}))
+	})
+
+	t.Run("non-private non-bearer is open", func(t *testing.T) {
+		assert.NoError(t, checkPeerGroupAccess(&service.Service{}, nil))
+	})
+}