fix(management): private-service validation + tunnel-IP lookup semantics

- Require an explicit port for L4 cluster targets. validateL4Target
  exempted TargetTypeCluster from the port check, but buildPathMappings
  serializes every L4 target via net.JoinHostPort(host, port) — port=0
  shipped a ":0" upstream. Cluster targets use the same Host/Port
  fields, so the same requirement applies.
- GetPeerByIP returns NotFound on a tunnel-IP miss instead of mapping
  every error to Internal. The proxy's ValidateTunnelPeer probes IPs
  that legitimately aren't in the roster; the miss is expected and now
  distinguishable from a real store failure.
- Thread ctx into getClusterCapability's gorm query so a cancelled
  request doesn't keep the store busy.

Tests updated for the L4-cluster port requirement and the GetPeerByIP
NotFound path.

management/internals/modules/reverseproxy/service/service.go

@@ -932,7 +932,11 @@ func (s *Service) validateL4Target(target *Target) error {
 	if target.TargetId == "" {
 		return errors.New("target_id is required for L4 services")
 	}
-	if target.TargetType != TargetTypeCluster && target.Port == 0 {
+	// Cluster targets resolve their upstream host:port from the target's
+	// own Host/Port fields just like the other L4 types — buildPathMappings
+	// emits net.JoinHostPort(target.Host, target.Port) for every L4
+	// target, so allowing port=0 here would let ":0" reach the proxy.
+	if target.Port == 0 {
 		return errors.New("target port is required for L4 services")
 	}
 	switch target.TargetType {

management/internals/modules/reverseproxy/service/service_test.go

@@ -1176,7 +1176,12 @@ func TestValidate_HTTPClusterTarget_RequiresDirectUpstream(t *testing.T) {
 	assert.ErrorContains(t, rp.Validate(), "direct upstream disabled", "cluster target must reject direct_upstream=false")
 }
 
-func TestValidate_L4ClusterTarget(t *testing.T) {
+// TestValidate_L4ClusterTarget_RequiresPort confirms that an L4 cluster
+// target without an explicit port is rejected. buildPathMappings emits
+// net.JoinHostPort(target.Host, target.Port) for every L4 target — so
+// allowing port=0 would let the proxy ship ":0" upstreams. The port
+// requirement is the same as every other L4 target type.
+func TestValidate_L4ClusterTarget_RequiresPort(t *testing.T) {
 	rp := validProxy()
 	rp.Mode = ModeTCP
 	rp.ListenPort = 9000
@@ -1186,7 +1191,12 @@ func TestValidate_L4ClusterTarget(t *testing.T) {
 		Protocol:   "tcp",
 		Enabled:    true,
 	}}
-	require.NoError(t, rp.Validate(), "L4 cluster target must validate without an explicit port")
+	assert.ErrorContains(t, rp.Validate(), "port is required",
+		"L4 cluster target must require an explicit port like other L4 target types")
+
+	rp.Targets[0].Port = 5432
+	rp.Targets[0].Host = "db.lan"
+	require.NoError(t, rp.Validate(), "L4 cluster target with host:port must validate")
 }
 
 func TestService_Copy_RoundtripsPrivate(t *testing.T) {