Drop wkhtmltopdf so the base image can move to Debian trixie #35

Current python:3.13 tags are built on Debian trixie, which no longer packages wkhtmltopdf, so the image build fails:

E: Package 'wkhtmltopdf' has no installation candidate

This keeps the image on bookworm and blocks base image updates (#31). wkhtmltopdf is unmaintained upstream, and nothing in the repo uses it since pdfkit was removed. Playwright's page.pdf() is already in the image for HTML to PDF.

Proposal: remove it from the Dockerfile and move to trixie. Some libraries in the apt list were renamed in trixie (e.g. libasound2 -> libasound2t64) and will need the same check.