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/.env.example

@@ -0,0 +1,17 @@
+# Copy to deploy/.env, restrict it (chmod 600), and fill in. deploy/.env is git-ignored
+# and never goes into an image. How it reaches sahaquiel is decided when the server is
+# set up (e.g. rendered from Proton Pass on the deploying machine and copied over SSH,
+# so the server itself never holds Proton credentials).
+
+# `http://localhost` for the local rehearsal; `learn.halkyon.eu` on sahaquiel.
+DOMAIN=http://localhost
+
+# Postgres password, shared by the postgres and server services.
+# Generate: openssl rand -base64 32
+POSTGRES_PASSWORD=
+
+# Pepper for hashing session keys (server-side sessions). NEVER change it once
+# sessions exist: everyone would be signed out, and anonymous accounts can't sign
+# back in. Rotation only via fallbackSessionKeyHashPeppers.
+# Generate: openssl rand -base64 32
+SESSION_KEY_HASH_PEPPER=