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/script/build

@@ -1,6 +1,6 @@
 #!/bin/sh
 # script/build: compile the static netwatch-server binary into the
-# backend project root, with its version and architecture stamped in.
+# backend project root, with its version stamped in.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
@@ -14,7 +14,7 @@ main() {
     version="${VERSION:-$(git describe --always --dirty 2>/dev/null || echo dev)}"
 
     CGO_ENABLED=0 go build -trimpath \
-        -ldflags "-s -w -X main.Version=$version -X main.Buildarch=$(uname -m)" \
+        -ldflags "-s -w -X main.Version=$version" \
         -o netwatch-server ./cmd/netwatch-server/
 }