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.

Gemfile

@@ -12,6 +12,12 @@ gem 'erubi',           '~> 1.13'
 gem 'mail',            '~> 2.8'
 gem 'dotenv',          '~> 3.1'
 
+gem 'pg',              '~> 1.5'
+gem 'sequel',          '~> 5.86'
+gem 'prawn',           '~> 2.5'
+gem 'prawn-table',     '~> 0.2'
+gem 'aws-sdk-s3',      '~> 1.170'
+
 group :development do
   gem 'rerun', '~> 0.14'
 end