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>

deploy/home-jellyfin/wg0.conf.example

@@ -0,0 +1,17 @@
+# The home end of the tunnel to the instance's server (ADR 0032), copied to
+# wireguard/wg_confs/wg0.conf (git-ignored) with its two keys filled in. Setting it up:
+# ../sidecars/README.md, *Jellyfin at home*.
+
+[Interface]
+Address = 10.61.96.2/32
+PrivateKey = <the home end's private key>
+
+[Peer]
+PublicKey = <the instance's public key>
+Endpoint = learn.halkyon.eu:51820
+# The instance's end alone, through which Caddy's requests come (masqueraded by Docker
+# on the instance's server): nothing else of the internet goes through the tunnel.
+AllowedIPs = 10.61.96.1/32
+# Dialling out keeps the way open through the router and brings the tunnel back within
+# 25 s after the machine wakes (D10).
+PersistentKeepalive = 25