Initial commit: shared invoice/correction-invoice PDF generation

Consolidates code previously hand-duplicated between the einfach-produktiv
frontend and payload backend repos into one canonical implementation,
consumed as a git dependency by both instead of being kept in sync by eye.

While merging the two correction-invoice copies, found and fixed three
real drifts between them:
- variantName was silently dropped in the backend's emailed Stornorechnung/
  Gutschrift, but present in the frontend's re-download copy
- the backend's correction-invoice footer wasn't position:fixed, unlike
  the original invoice and the frontend's copy
- the backend used a numeric date format (03.07.2025) while the original
  invoice and the frontend's copy both used a spelled-out month (03. Juli
  2025) — a re-download didn't visually match what was emailed

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitignore

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+node_modules/