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

Dockerfile.backend did not follow the Go multistage lint-stage pattern
REPO_POLICIES.md mandates, and dragged the whole git history into the
build context to resolve a version string.

- Add an `AS lint` stage on the hash-pinned golangci/golangci-lint
  image (v2.7.2, the same golangci-lint commit main already pins), which
  ships Go, gofmt, make and the linter, so nothing is installed in it.
  It runs `make fmt-check` then `make lint`.
- Add `COPY --from=lint /src/go.sum /dev/null` to the build stage so
  BuildKit cannot run the two stages in parallel and let a lint failure
  through.
- Stop compiling golangci-lint from source in the build stage.
- Drop `COPY .git /repo/.git`; the version now comes from
  `ARG VERSION=dev`, passed to the build via `make build VERSION=...`.
- Drop gcc and musl-dev, and the corresponding
  `-linkmode external -extldflags -static` in backend/Makefile. The
  build is now `CGO_ENABLED=0 go build -trimpath` with
  `-ldflags "-s -w -X main.Version=... -X main.Buildarch=..."`, which is
  static without a C toolchain.
- backend/Makefile's VERSION is now overridable and degrades to `dev`
  when git or .git is unavailable instead of emitting a git error and
  building an empty version string.
- Every FROM stays pinned by @sha256 with a version and date comment.

Runtime stage, exposed port and entrypoint are unchanged.

.gitignore

@@ -1,27 +1,4 @@
-# OS
-.DS_Store
-Thumbs.db
-
-# Editors
-*.swp
-*.swo
-*~
-*.bak
-.idea/
-.vscode/
-*.sublime-*
-
-# Node
 node_modules/
-
-# Environment / secrets
-.env
-.env.*
-*.pem
-*.key
-
-# Build output
 dist/
-
+.DS_Store
-# Logs
 *.log

TODO.md

@@ -26,13 +26,6 @@ files, so merging it also closes most compliance gaps.
   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
-  repo's `dist/` and `*.log`. `.env`, `.env.*`, `*.pem`, and `*.key` are now
-  ignored repo-wide, not just under `backend/`. Excluding `.git` from
-  `.dockerignore` stays deferred: both images read git metadata at build time
-  (`COPY .git` in `Dockerfile.backend`, `git rev-parse` in `vite.config.js`)
 - 2026-07-07 Adopted scripts-to-rule-them-all: `script/` entrypoints, Makefile
   shims, README Entrypoints section
 - 2026-02-27: backend with buffered zstd-compressed report storage; CI workflow