build: Dockerfile.backend multistage lint stage (closes #17)

TODO.md

@@ -22,6 +22,10 @@ files, so merging it also closes most compliance gaps.
 
 # Completed Steps
 
+- 2026-08-09: `Dockerfile.backend` reworked to the mandated Go multistage
+  lint-stage pattern: separate `lint` stage on the hash-pinned
+  `golangci/golangci-lint` image, `COPY --from=lint` stage dependency,
+  `CGO_ENABLED=0` static build driven by `ARG VERSION`, and no more `COPY .git`
 - 2026-08-09: dotfile compliance — lifted `backend/.editorconfig` to the repo
   root so `root = true` covers the frontend too, and replaced `.gitignore` with
   the org model (OS, editor, node, and environment/secrets sections) plus this