[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.

management/internals/modules/reverseproxy/sessionkey/sessionkey.go

@@ -20,6 +20,20 @@ type KeyPair struct {
 type Claims struct {
 	jwt.RegisteredClaims
 	Method auth.Method `json:"method"`
+	// Email is the calling user's email address. Carried so the
+	// proxy can stamp identity on upstream requests (e.g.
+	// x-litellm-end-user-id) without an extra management
+	// round-trip on every cookie-bearing request.
+	Email string `json:"email,omitempty"`
+	// Groups carries the user's group IDs so the proxy can stamp them
+	// onto upstream requests (X-NetBird-Groups) from the cookie path
+	// without an extra management round-trip.
+	Groups []string `json:"groups,omitempty"`
+	// GroupNames carries the human-readable display names for the ids
+	// in Groups, ordered identically (positional pairing). Slice may be
+	// shorter than Groups for tokens minted before names were
+	// resolvable; the consumer falls back to ids for missing positions.
+	GroupNames []string `json:"group_names,omitempty"`
 }
 
 func GenerateKeyPair() (*KeyPair, error) {
@@ -34,7 +48,13 @@ func GenerateKeyPair() (*KeyPair, error) {
 	}, nil
 }
 
-func SignToken(privKeyB64, userID, domain string, method auth.Method, expiration time.Duration) (string, error) {
+// SignToken mints a session JWT for the given user and domain. email,
+// groups, and groupNames, when non-empty, are embedded so the proxy can
+// authorise and stamp identity for policy-aware middlewares without a
+// management round-trip on every cookie-bearing request. groupNames
+// pairs positionally with groups; pass nil when names couldn't be
+// resolved.
+func SignToken(privKeyB64, userID, email, domain string, method auth.Method, groups, groupNames []string, expiration time.Duration) (string, error) {
 	privKeyBytes, err := base64.StdEncoding.DecodeString(privKeyB64)
 	if err != nil {
 		return "", fmt.Errorf("decode private key: %w", err)
@@ -56,7 +76,10 @@ func SignToken(privKeyB64, userID, domain string, method auth.Method, expiration
 			IssuedAt:  jwt.NewNumericDate(now),
 			NotBefore: jwt.NewNumericDate(now),
 		},
-		Method: method,
+		Method:     method,
+		Email:      email,
+		Groups:     append([]string(nil), groups...),
+		GroupNames: append([]string(nil), groupNames...),
 	}
 
 	token := jwt.NewWithClaims(jwt.SigningMethodEdDSA, claims)