Drive the pipeline through make, the way riksdata and rd-web do

Both workflows restated the build in YAML: gofmt, go vet and go test inline
in two places, buildx inline in a third, and the chart's version sed'd into
Chart.yaml before packaging. The Makefile added in 1081260 then described
the same checks a second time for local use, which made "green locally means
green in CI" a promise about keeping two files in step rather than a
property of the setup.

riksdata and rd-web never had that problem — their workflows call make and
have done all along. This repo was the odd one out, and only because it had
no Makefile until today. Now ci.yaml runs `make fmt lint test`, release.yaml
runs the same plus `make binaries`, `make push`, `make helm-package` and
`make helm-push`, and the reasoning behind each check lives on the target
rather than in whichever YAML file was edited last.

Three things change rather than just move:

The chart is linted before it is published. release.yaml packaged and pushed
without ever rendering the templates, so a chart that did not compile would
have reached the registry and been found by Flux. ci.yaml gained a chart job
for the same reason.

helm package --version --app-version replaces the sed. The published
metadata is identical, but the tree is no longer mutated mid-build, and it
is what the rest of the release process already assumed happened.

`make push` refuses VERSION=dev. Publishing is one command now, so it is
also one command to run by accident; dev is not a version anyone releases.

Deliberately not moved: uploading the release assets. Compiling them is
`make binaries` and runs anywhere, but the upload needs a token and the
Gitea release API, which is the workflow's business and not something worth
a target.

`push` builds and pushes in one step, unlike riksdata's separate build and
push, because buildx cannot load a multi-platform image into the local store
— it can only push it. `build` stays single-platform and local-only.

Claude-Session: https://claude.ai/code/session_01S7R4gWTz5wh5xCY4nCSJjN

.gitignore

@@ -1,6 +1,11 @@
 # build output
 /terdut
 /terdut-server
+# `make binaries` and `make helm-package` write here
+/dist/
+# isolated helm repo list written by the publishing targets, so the machine-wide
+# one (which has an unreachable entry) cannot abort a release
+/.helm-repos.yaml
 
 # SQLite database files
 *.db