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/internal/globals/globals.go

@@ -10,22 +10,18 @@ var (
 	Appname string
 	// Version is the git version tag.
 	Version string
-	// Buildarch is the build architecture.
-	Buildarch string
 )
 
 // Globals holds build-time metadata for the application.
 type Globals struct {
-	Appname   string
-	Version   string
-	Buildarch string
+	Appname string
+	Version string
 }
 
 // New creates a Globals instance from package-level variables.
 func New(_ fx.Lifecycle) (*Globals, error) {
 	return &Globals{
-		Appname:   Appname,
-		Buildarch: Buildarch,
-		Version:   Version,
+		Appname: Appname,
+		Version: Version,
 	}, nil
 }