Remove Buildarch: read the architecture at run time (closes #83)

The architecture is no longer passed in at build time. The Buildarch
variable and field are gone from main and globals, script/build no
longer stamps it in with -X, and the startup and listen log lines
report runtime.GOARCH under the key "arch". The Dockerfile comment and
backend/README.md no longer describe an architecture being stamped in.

Model: opus-5-5

backend/cmd/netwatch-server/main.go

@@ -19,9 +19,8 @@ import (
 
 //nolint:gochecknoglobals // set via ldflags at build time
 var (
-	Appname   = "netwatch-server"
-	Version   string
-	Buildarch string
+	Appname = "netwatch-server"
+	Version string
 )
 
 func main() {
@@ -40,7 +39,6 @@ func main() {
 
 	globals.Appname = Appname
 	globals.Version = Version
-	globals.Buildarch = Buildarch
 
 	fx.New(
 		fx.Provide(