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/server/http.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"net"
 	"net/http"
+	"runtime"
 	"strconv"
 	"time"
 
@@ -53,7 +54,7 @@ func (s *Server) listenAndServe() {
 	s.log.Info("http begin listen",
 		"listenaddr", s.httpServer.Addr,
 		"version", s.params.Globals.Version,
-		"buildarch", s.params.Globals.Buildarch,
+		"arch", runtime.GOARCH,
 	)
 
 	err := s.httpServer.ListenAndServe()