feat(deploy): Jellyfin at home, published through WireGuard (ADR 0032)

The instance serves jellyfin.halkyon.eu and its Caddy proxies through a
WireGuard tunnel to a Jellyfin on the operator's own machine, which dials
out, so nothing names the home network's address and its router opens no
port. compose.jellyfin-home.yaml takes the sidecar's place, with its
site's rules unchanged (jellyfin.caddy's upstream is now a setting), and
leaves JELLYFIN_URL unset, so the instance suggests it only while it
answers.

On the instance's server the tunnel reaches Jellyfin from Caddy alone and
nothing of the host (wg-jellyfin-rules.sh, set and removed with the
tunnel); the CI runner's unit denies it, and CI's Sandbox step fails if a
job reaches it. The home end keeps Jellyfin in WireGuard's network
namespace, its port on the loopback alone. The steps are in
deploy/sidecars/README.md, the rules held by test/deploy/jellyfin_home_test.dart.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

.forgejo/workflows/ci.yaml

@@ -55,6 +55,10 @@ jobs:
           if timeout 5 bash -c 'exec 3<>/dev/tcp/fd4a:6c6b:7e80::2/443' 2>/dev/null; then
             fail "Caddy's IPv6 network is reachable"
           fi
+          # Nor is Jellyfin at home through the host's tunnel, where it is up (ADR 0032, D6).
+          if timeout 5 bash -c 'exec 3<>/dev/tcp/10.61.96.2/8096' 2>/dev/null; then
+            fail "the tunnel to Jellyfin at home is reachable"
+          fi
           echo "This job can neither see nor reach production."
 
       # A job cancelled, or stopped at timeout-minutes, is killed with its step's process