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>

.dockerignore

@@ -0,0 +1,20 @@
+# The Docker build context is the repository root. Nothing secret, local or bulky
+# goes into it: the server's Dockerfile copies the whole halkyon_learn_server folder.
+
+# Secrets and local state
+halkyon_learn_server/config/passwords.yaml
+halkyon_learn_server/.serverpod/
+deploy/.env
+**/*.log
+
+# Tooling and build output
+.git/
+**/.dart_tool/
+**/build/
+.vscode/
+.claude/
+
+# Not needed to build the server
+halkyon_learn_flutter/
+docs/
+NEXT.md