docs: add CONTRIBUTING and adopt Conventional Commits

A dozen conventions had accumulated that existed only in my head and in the
commit log: Finnish interface against English code, no infrastructure detail
in the repository, migrations immutable once shipped, interactions that patch
rather than navigate, ponytail comments marking deliberate shortcuts, and
asserting what a response does rather than only that it responded — the one
that let the /ruoat 404 ship.

Commit messages take a type from here on: feat, fix, chore and the rest. The
body still carries the weight, since these commits are the only design record
this project has. Pull request titles get the same treatment, because main is
squash-merged and a title becomes a commit message on it.

README.md

@@ -58,6 +58,9 @@ One static Go binary. No Node.js, no bundler, no separate database server.
 | Auth | HTTP Basic, one shared household password |
 | Runtime image | `FROM scratch` |
 
+Working on it: [CONTRIBUTING.md](CONTRIBUTING.md) — branches, commit messages,
+and the conventions that are easy to miss.
+
 ## Branches
 
 `main` holds released versions only. Every release tag points at a commit on