fix(docker): use ubuntu:24.04 runtime instead of debian:bookworm-slim (#44)

* fix(docker): add PostgreSQL APT repo for postgresql-client-16

Debian Bookworm default repos only ship PostgreSQL 15. The Docker
runtime stage needs postgresql-client-16 for the entrypoint script,
so add the official PGDG APT repository.

- Add PGDG GPG key and sources.list entry for bookworm-pgdg
- Install ca-certificates and curl first (needed for repo setup)
- Purge gnupg2 after use to keep image lean
- Verify argon2 package name is correct for Bookworm (it is)

* fix(docker): use ubuntu:24.04 runtime instead of debian:bookworm-slim

The project targets Ubuntu 24.04, not Debian Bookworm. Ubuntu 24.04
includes PostgreSQL 16 in default repos, eliminating the need for the
PGDG APT repo workaround. Also fixes libssl3 → libssl3t64 package name
for the time64 transition in Ubuntu 24.04.

Dockerfile

@@ -58,11 +58,11 @@ RUN npm run build
 # ---------------------------------------------------------------------------
 # Stage 3: Runtime
 # ---------------------------------------------------------------------------
-FROM debian:bookworm-slim AS runtime
+FROM ubuntu:24.04 AS runtime
 
 RUN apt-get update && apt-get install -y \
     ca-certificates \
-    libssl3 \
+    libssl3t64 \
     libfontconfig1 \
     postgresql-client-16 \
     argon2 \