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/server/store/sql_store.go

@@ -2178,7 +2178,8 @@ func (s *SqlStore) getServices(ctx context.Context, accountID string) ([]*rpserv
 	const serviceQuery = `SELECT id, account_id, name, domain, enabled, auth,
 		meta_created_at, meta_certificate_issued_at, meta_status, proxy_cluster,
 		pass_host_header, rewrite_redirects, session_private_key, session_public_key,
-		mode, listen_port, port_auto_assigned, source, source_peer, terminated
+		mode, listen_port, port_auto_assigned, source, source_peer, terminated,
+		private, access_groups
 		FROM services WHERE account_id = $1`
 
 	const targetsQuery = `SELECT id, account_id, service_id, path, host, port, protocol,
@@ -2193,10 +2194,11 @@ func (s *SqlStore) getServices(ctx context.Context, accountID string) ([]*rpserv
 	services, err := pgx.CollectRows(serviceRows, func(row pgx.CollectableRow) (*rpservice.Service, error) {
 		var s rpservice.Service
 		var auth []byte
+		var accessGroups []byte
 		var createdAt, certIssuedAt sql.NullTime
 		var status, proxyCluster, sessionPrivateKey, sessionPublicKey sql.NullString
 		var mode, source, sourcePeer sql.NullString
-		var terminated, portAutoAssigned sql.NullBool
+		var terminated, portAutoAssigned, private sql.NullBool
 		var listenPort sql.NullInt64
 		err := row.Scan(
 			&s.ID,
@@ -2219,6 +2221,8 @@ func (s *SqlStore) getServices(ctx context.Context, accountID string) ([]*rpserv
 			&source,
 			&sourcePeer,
 			&terminated,
+			&private,
+			&accessGroups,
 		)
 		if err != nil {
 			return nil, err
@@ -2230,6 +2234,16 @@ func (s *SqlStore) getServices(ctx context.Context, accountID string) ([]*rpserv
 			}
 		}
 
+		if len(accessGroups) > 0 {
+			if err := json.Unmarshal(accessGroups, &s.AccessGroups); err != nil {
+				return nil, fmt.Errorf("unmarshal access_groups: %w", err)
+			}
+		}
+
+		if private.Valid {
+			s.Private = private.Bool
+		}
+
 		s.Meta = rpservice.Meta{}
 		if createdAt.Valid {
 			s.Meta.CreatedAt = createdAt.Time
@@ -5826,6 +5840,7 @@ var validCapabilityColumns = map[string]struct{}{
 	"supports_custom_ports": {},
 	"require_subdomain":     {},
 	"supports_crowdsec":     {},
+	"private":               {},
 }
 
 // GetClusterSupportsCustomPorts returns whether any active proxy in the cluster
@@ -5840,6 +5855,12 @@ func (s *SqlStore) GetClusterRequireSubdomain(ctx context.Context, clusterAddr s
 	return s.getClusterCapability(ctx, clusterAddr, "require_subdomain")
 }
 
+// GetClusterSupportsPrivate reports whether any active proxy in the cluster
+// has the private capability (nil = unreported).
+func (s *SqlStore) GetClusterSupportsPrivate(ctx context.Context, clusterAddr string) *bool {
+	return s.getClusterCapability(ctx, clusterAddr, "private")
+}
+
 // GetClusterSupportsCrowdSec returns whether all active proxies in the cluster
 // have CrowdSec configured. Returns nil when no proxy reported the capability.
 // Unlike other capabilities that use ANY-true (for rolling upgrades), CrowdSec