[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/server/store/store.go

@@ -312,6 +312,7 @@ type Store interface {
 	GetClusterSupportsCustomPorts(ctx context.Context, clusterAddr string) *bool
 	GetClusterRequireSubdomain(ctx context.Context, clusterAddr string) *bool
 	GetClusterSupportsCrowdSec(ctx context.Context, clusterAddr string) *bool
+	GetClusterSupportsPrivate(ctx context.Context, clusterAddr string) *bool
 	CleanupStaleProxies(ctx context.Context, inactivityDuration time.Duration) error
 	GetProxyByAccountID(ctx context.Context, accountID string) (*proxy.Proxy, error)
 	CountProxiesByAccountID(ctx context.Context, accountID string) (int64, error)
@@ -320,9 +321,38 @@ type Store interface {
 
 	GetCustomDomainsCounts(ctx context.Context) (total int64, validated int64, err error)
 
+	// GetProxyMetrics returns aggregated proxy / cluster counts for the
+	// self-hosted metrics worker. Self-hosted only — file-based stores
+	// return a zero-valued struct.
+	GetProxyMetrics(ctx context.Context) (ProxyMetrics, error)
+
 	GetRoutingPeerNetworks(ctx context.Context, accountID, peerID string) ([]string, error)
 }
 
+// ProxyMetrics aggregates self-hosted proxy + cluster usage signals
+// surfaced to the telemetry payload. Each field is best-effort: when a
+// store cannot answer (e.g. FileStore) all fields are zero.
+type ProxyMetrics struct {
+	// Clusters counts distinct cluster_address values across the proxies
+	// table — every cluster the management server has heard from, online or not.
+	Clusters int64
+	// ClustersBYOP counts distinct cluster_address values that are owned
+	// by an account (account_id IS NOT NULL). These are bring-your-own-proxy
+	// installations as opposed to NetBird-operated shared clusters.
+	ClustersBYOP int64
+	// ClustersPrivate counts distinct cluster_address values where at
+	// least one proxy reported the private capability (embedded
+	// `netbird proxy` running inside a client).
+	ClustersPrivate int64
+	// Proxies is the total number of proxy rows currently persisted.
+	Proxies int64
+	// ProxiesConnected is the subset of proxies whose status is
+	// "connected" AND last_seen falls within the active heartbeat window
+	// (~2 * heartbeat interval). Proxies the controller hasn't pruned
+	// yet but that are visibly stale don't count.
+	ProxiesConnected int64
+}
+
 const (
 	postgresDsnEnv       = "NB_STORE_ENGINE_POSTGRES_DSN"
 	postgresDsnEnvLegacy = "NETBIRD_STORE_ENGINE_POSTGRES_DSN"