gems: add pg, sequel, prawn, aws-sdk-s3 for invoicing

- pg + sequel: connect to the storage stack's Postgres.
- prawn + prawn-table: render invoice PDFs.
- aws-sdk-s3: upload PDFs to MinIO and mint presigned download URLs.

Also pulls libpq-dev into the Docker build (for the pg gem) and the
postgresql client lib into the Nix dev shell.

Dockerfile

@@ -2,6 +2,7 @@ FROM ruby:3.4-slim-bookworm
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
     build-essential \
+    libpq-dev \
  && rm -rf /var/lib/apt/lists/*
 
 RUN groupadd -g 11000 asxpio \