policy/v2: map no numeric email local part to a login

The client looks a numeric login up as a UID, so 0@domain reached root
under a localpart rule, and another number someone else's account.

hscontrol/policy/v2/filter.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"net/netip"
 	"slices"
+	"strconv"
 	"strings"
 	"sync"
 	"time"
@@ -659,6 +660,13 @@ func resolveLocalparts(
 				continue
 			}
 
+			// The client looks a numeric login up as a UID, so 0@domain would
+			// reach root and another number someone else's account.
+			_, notNumeric := strconv.ParseUint(localpart, util.Base10, 32)
+			if notNumeric == nil {
+				continue
+			}
+
 			result[user.ID] = localpart
 		}
 	}

hscontrol/policy/v2/filter_test.go

@@ -3112,6 +3112,17 @@ func TestResolveLocalparts(t *testing.T) {
 			},
 			want: map[uint]string{1: "dave+ssh"},
 		},
+		{
+			name:    "numeric local part is not a login",
+			entries: []SSHUser{"localpart:*@example.com"},
+			users: types.Users{
+				{Name: "zero", Email: "0@example.com", ID: 1},
+				{Name: "zeros", Email: "00@example.com", ID: 2},
+				{Name: "employee", Email: "1042@example.com", ID: 3},
+				{Name: "mixed", Email: "1042a@example.com", ID: 4},
+			},
+			want: map[uint]string{4: "1042a"},
+		},
 	}
 
 	for _, tt := range tests {