The deploy workflow is now a single job that builds, pushes, and
deploys in one runner. Tag computation moved to docker/metadata-action,
the per-deploy .env perl rewrite is gone (host pins LIBRENOTES_IMAGE
once; main pushes update :main rolling, releases pin to :vX.Y.Z by
manual edit), and both jobs run in our bespoke runner image whose
runner user already has socket access via group membership.
ci.yml moves to the same image so go/make/node are all available
without per-step apt installs.
Drops compose.prod.yaml (unused, redundant with compose.netcup.yaml).
compose.netcup.yaml now references ${LIBRENOTES_IMAGE} with pull_policy: always
and resets the base build context. .env.netcup.example documents the new
LIBRENOTES_IMAGE key (default git.librete.ch/public/librenotes:main, pin to
immutable tag for prod). Rollback: edit .env LIBRENOTES_IMAGE + up -d.
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