ci(deploy): switch to libretech/runner-image:v1 and consolidate

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).

docs/operations.md

@@ -51,14 +51,16 @@ docker compose -f compose.yaml -f compose.netcup.yaml up -d
 
 ### Rollback
 
-Image tags are immutable per commit / version. To roll back, set
-`LIBRENOTES_IMAGE` to the previous tag in `.env` and run the same
-`up -d` command. The deployment workflow does not auto-rollback
-on health-check failure — failed health alerts the operator via
-the workflow itself, who can then redeploy the prior tag manually.
+Tag pushes publish immutable tags (`:vX.Y.Z`). To pin or roll back,
+edit `LIBRENOTES_IMAGE` in `/srv/librenotes/.env` and re-run pull +
+up. The deployment workflow itself never rewrites the file — failed
+health checks abort the workflow and the operator redeploys
+manually.
 
 ```sh
 # Example: roll back to v0.1.0
+ssh netcup
+cd /srv/librenotes
 perl -i -pe 's|^LIBRENOTES_IMAGE=.*|LIBRENOTES_IMAGE=git.librete.ch/public/librenotes:v0.1.0|' .env
 docker compose -f compose.yaml -f compose.netcup.yaml pull
 docker compose -f compose.yaml -f compose.netcup.yaml up -d
@@ -67,17 +69,17 @@ docker compose -f compose.yaml -f compose.netcup.yaml up -d
 ### Gitea Actions runner
 
 Workflows run on the netcup `act_runner` (see `runner/` stack in
-the netcup umbrella). Both jobs declare `container: catthehacker/ubuntu:runner-latest`
-because:
+the netcup umbrella). Both `ci.yml` and `deploy.yml` declare
+`container: image: git.librete.ch/libretech/runner-image:v1` —
+a bespoke Ubuntu 24.04 image (built and signed by us, hosted on
+the same Gitea instance) that bundles `git`, `make`, `node`,
+`perl`, `ssh`, and a docker CLI. The image's `runner` user is
+pre-joined to `docker` group gid 998 so the auto-mounted
+`/var/run/docker.sock` is writable without `--user root`.
 
-- The default runner label image (`node:20-bookworm`) lacks `make`
-  and `docker`, both required by the workflows.
-- The `runner-latest` image bundles `make`, `git`, `curl`, `ssh`,
-  `node`, plus a docker CLI.
-
-The runner config (`runner/config.yaml`) declares
-`/var/run/docker.sock` as a `valid_volume` so the build job can
-mount the host socket and push images via `docker/build-push-action`.
+The runner config (`runner/config.yaml`) whitelists
+`/var/run/docker.sock` under `valid_volumes` to allow the
+auto-mount.
 
 ## Backups