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/config/config.go

@@ -28,6 +28,7 @@ type Config struct {
 	AdminToken         string
 	AdminUser          string
 	AdminPassword      string
+	AdminAllowRemote   bool
 	CORSAllowedOrigins []string
 	TokenDefaultTTL    time.Duration
 
@@ -50,6 +51,10 @@ func Load() (Config, error) {
 	if err != nil {
 		return Config{}, fmt.Errorf("parse AUTO_SCAN_ON_START: %w", err)
 	}
+	adminAllowRemote, err := boolEnv("ADMIN_ALLOW_REMOTE", false)
+	if err != nil {
+		return Config{}, fmt.Errorf("parse ADMIN_ALLOW_REMOTE: %w", err)
+	}
 	tokenDefaultTTL, err := durationEnv("TOKEN_DEFAULT_TTL", 720*time.Hour)
 	if err != nil {
 		return Config{}, fmt.Errorf("parse TOKEN_DEFAULT_TTL: %w", err)
@@ -76,6 +81,7 @@ func Load() (Config, error) {
 		AdminToken:          strings.TrimSpace(stringEnv("ADMIN_TOKEN", "")),
 		AdminUser:           strings.TrimSpace(stringEnv("ADMIN_USER", "")),
 		AdminPassword:       stringEnv("ADMIN_PASSWORD", ""),
+		AdminAllowRemote:    adminAllowRemote,
 		CORSAllowedOrigins:  listEnv("CORS_ALLOWED_ORIGINS"),
 		TokenDefaultTTL:     tokenDefaultTTL,
 		HealthCheckInterval: healthCheckInterval,