ci: bound runner disk + bust the disk-full-corrupted cargo target cache

The self-hosted runner filled its disk (95%, builds failing on ENOSPC): every CI
push builds a sha-<commit>-tagged Docker image per pipeline, and since those tags
are never dangling a plain `docker image prune` skips them — they piled up to 589
images / ~85 GB plus 18 GB of build cache. Two parts:

- scripts/ci/docker-prune.{service,timer}: a host-level systemd timer (every 6h,
  Persistent) that prunes images/build-cache/containers older than 24h — in-use
  images stay protected. Checked in (the runner is hand-provisioned and shared
  across orgs) and already installed live; reclaimed 89 GB -> 39 GB (95% -> 42%).

- ci.yml / deb.yml: bump the `cargo-target-<rustc>-*` cache key to `-v2-`. The
  disk-full build let actions/cache save a truncated target/ (a dep's .rmeta went
  missing -> "error[E0463]: can't find crate for pem_rfc7468" while compiling der).
  A suffix bump is useless here — restore-keys would fall back to the poisoned
  prefix — so the prefix is versioned to force one clean rebuild. cargo-home is
  untouched (sources were intact; the failure was a missing build artifact).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/ci/docker-prune.service

@@ -0,0 +1,28 @@
+# Docker disk hygiene for the self-hosted Gitea Actions runner (home-runner-1, 192.168.1.52).
+#
+# Why this exists: every CI push builds and sha-<commit>-tags a Docker image per pipeline
+# (rust-ci, web, docs, fedora-rpm, fedora44-rpm, ...). Those tags are never dangling, so a
+# plain `docker image prune` SKIPS them and they accumulate forever — that is what filled the
+# disk (589 images / ~85 GB, builds failing on ENOSPC). This trims everything older than 24h;
+# images IN USE by a running container are always protected regardless of age.
+#
+# Host-level, not per-repo CI, because the runner is shared (punktfunk + other orgs all benefit).
+#
+# Install on the runner host (root):
+#   cp scripts/ci/docker-prune.{service,timer} /etc/systemd/system/
+#   systemctl daemon-reload && systemctl enable --now docker-prune.timer
+# See also scripts/ci/setup-macos-runner.sh for the macOS runner.
+
+[Unit]
+Description=Prune aged Docker images / build cache (CI runner disk hygiene)
+Documentation=https://git.unom.io/unom/punktfunk
+Wants=docker.service
+After=docker.service
+
+[Service]
+Type=oneshot
+# '-' prefix: each step is independent — a no-op/failure never blocks the others.
+ExecStart=-/usr/bin/docker image prune -af --filter until=24h
+ExecStart=-/usr/bin/docker builder prune -af --filter until=24h
+ExecStart=-/usr/bin/docker buildx prune -af --filter until=24h
+ExecStart=-/usr/bin/docker container prune -f --filter until=24h

scripts/ci/docker-prune.timer

@@ -0,0 +1,13 @@
+# Runs docker-prune.service every 6h. Persistent=true catches up after downtime.
+# Install: see the header of docker-prune.service.
+
+[Unit]
+Description=Run docker-prune every 6h (CI runner disk hygiene)
+
+[Timer]
+OnCalendar=*-*-* 00/6:00:00
+RandomizedDelaySec=600
+Persistent=true
+
+[Install]
+WantedBy=timers.target