feat(deploy): sidecars beside the instance, Jellyfin the first, behind an edge network

An instance may now run services beside itself, each a compose file of
its own switched on in deploy/.env (COMPOSE_FILE=compose.yaml:compose.
jellyfin.yaml), so the deploy runs them with the instance. The first is
Jellyfin, which people watch and listen to learn from: the instance then
suggests it (JELLYFIN_URL from JELLYFIN_DOMAIN), and Caddy serves its
site, refusing every change to a user from outside, so a passwordless
demo account stays everyone's; its admin signs in through an SSH tunnel.

The instance's compose gets a second network, `edge`, for Caddy (at a
fixed address, which sidecars trust as their proxy) and the server;
Postgres stays on the internal one. Caddy loses its needless way to
Postgres, and a sidecar never gains one. A sidecar publishes no port,
as the CI runner may reach 127.0.0.1, and runs as a user ID of its own.
Caddy imports the sidecars' sites from sidecars/, mounted by their
compose files (none without), and redeploy.sh recreates it when one
changes. sidecars/README.md has the steps.

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/Caddyfile

@@ -19,3 +19,7 @@
 		reverse_proxy server:8080
 	}
 }
+
+# The sites of the sidecars switched on (sidecars/README.md), which their compose files
+# mount into sidecars/; none without.
+import sidecars/*.caddy