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>

.gitea/workflows/ci.yml

@@ -42,8 +42,12 @@ jobs:
       - uses: actions/cache@v4
         with:
           path: target
-          key: cargo-target-${{ env.rustc }}-${{ hashFiles('Cargo.lock') }}
-          restore-keys: cargo-target-${{ env.rustc }}-
+          # -v2-: the prior `cargo-target-<rustc>-*` cache was poisoned when the runner ran
+          # out of disk mid-build and actions/cache saved a truncated target/ (a dep's .rmeta
+          # went missing -> E0463 "can't find crate"). A suffix bump wouldn't help — restore-keys
+          # would fall back to the poisoned prefix — so the prefix itself is versioned.
+          key: cargo-target-v2-${{ env.rustc }}-${{ hashFiles('Cargo.lock') }}
+          restore-keys: cargo-target-v2-${{ env.rustc }}-
 
       - name: Format
         run: cargo fmt --all --check

.gitea/workflows/deb.yml

@@ -54,8 +54,10 @@ jobs:
       - uses: actions/cache@v4
         with:
           path: target
-          key: cargo-target-${{ env.rustc }}-${{ hashFiles('Cargo.lock') }}
-          restore-keys: cargo-target-${{ env.rustc }}-
+          # -v2-: bypass a target cache poisoned by a disk-full build (see ci.yml). Shares the
+          # key with ci.yml so the release build reuses its clean artifacts.
+          key: cargo-target-v2-${{ env.rustc }}-${{ hashFiles('Cargo.lock') }}
+          restore-keys: cargo-target-v2-${{ env.rustc }}-
 
       - name: Build release host + client
         run: |