Keep the admin surface local while serving tiles on the LAN

Binding to 0.0.0.0 used to demand credentials because the admin console and
the tile routes shared one listener. That forced a choice between keeping the
console off the network and letting other machines fetch tiles.

Admin routes are now loopback-only by default: the service can bind 0.0.0.0 so
/tiles, /vectors, the catalog and the probes reach the LAN, while /admin and
/api/v1/admin answer 403 to anything arriving from another host. Credentials
are only required when ADMIN_ALLOW_REMOTE=true actually opens the console up,
which is the case worth guarding.

Tests cover both sides: a remote admin request is refused, a remote tile
request still reaches the token check.

services/api-go/internal/basemapapi/server.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"html/template"
 	"log"
+	"net"
 	"net/http"
 	"net/url"
 	"strconv"
@@ -23,6 +24,9 @@ type AdminAuth struct {
 	Token    string
 	User     string
 	Password string
+	// AllowRemote permits admin requests from other hosts. Off by default: the
+	// service can then serve tiles to the LAN while the console stays local.
+	AllowRemote bool
 }
 
 func (a AdminAuth) enabled() bool {
@@ -348,6 +352,10 @@ func (s *Server) routes() {
 // service is only reachable on loopback, which main enforces at startup.
 func (s *Server) requireAdmin(next http.HandlerFunc) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
+		if !s.admin.AllowRemote && !isLoopbackRequest(r) {
+			writeError(w, http.StatusForbidden, errors.New("admin endpoints are loopback-only; set ADMIN_ALLOW_REMOTE=true to accept remote callers"))
+			return
+		}
 		if !s.admin.enabled() {
 			next(w, r)
 			return
@@ -361,6 +369,20 @@ func (s *Server) requireAdmin(next http.HandlerFunc) http.HandlerFunc {
 	}
 }
 
+// isLoopbackRequest reports whether the request reached us over a loopback
+// address. RemoteAddr is the only trustworthy source here; X-Forwarded-For is
+// attacker-controlled unless a trusted proxy is in front.
+func isLoopbackRequest(r *http.Request) bool {
+	host, _, err := net.SplitHostPort(strings.TrimSpace(r.RemoteAddr))
+	if err != nil {
+		host = strings.TrimSpace(r.RemoteAddr)
+	}
+	ip := net.ParseIP(strings.Trim(host, "[]"))
+	if ip == nil {
+		return false
+	}
+	return ip.IsLoopback()
+}
 func (s *Server) adminAllows(r *http.Request) bool {
 	if token := strings.TrimSpace(s.admin.Token); token != "" {
 		header := strings.TrimSpace(r.Header.Get("Authorization"))