feat(deploy): Docker Compose stack for the instance

Caddy, the Serverpod server and PostgreSQL 16 in deploy/. Only Caddy publishes ports; the server and Postgres are reachable only inside the Compose network (Docker bypasses ufw, and the server listens on every interface). Secrets come from deploy/.env as environment variables; there is no production passwords.yaml, and .dockerignore keeps local secrets out of the build context.

The server image builds on Serverpod's template with an Ubuntu 24.04 runtime: complete glibc incl. libgcc_s for the SQLite libraries Serverpod 4 bundles, CA certificates, a non-root user, and exec so SIGTERM reaches the server. The scaffold's dev-only docker-compose.yaml is removed.

Verified locally: image builds, all native libraries resolve, no passwords.yaml in the image, migrations apply at start, Caddy serves 200, port 8080 unreachable from the host, Postgres in UTC.

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

@@ -0,0 +1,12 @@
+# Caddy forwards to the API server only. Insights (8081) and a web server (8082) don't
+# run in production; if Insights is ever re-enabled, reach it through an SSH tunnel,
+# never through this file.
+#
+# {$DOMAIN} is `learn.halkyon.eu` on sahaquiel, where Caddy obtains and renews a
+# Let's Encrypt certificate by itself, and `http://localhost` for the local rehearsal.
+{$DOMAIN} {
+	encode zstd gzip
+
+	# Serverpod's streaming methods use WebSockets; reverse_proxy upgrades them as-is.
+	reverse_proxy server:8080
+}