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.

flake.nix

@@ -18,6 +18,8 @@
           pkgs.openssl
           pkgs.zlib
           pkgs.libyaml
+          pkgs.postgresql.lib
+          pkgs.postgresql
         ];
 
         shellHook = ''