Compress PDF export delivery by returning ZIP payload

[Copilot]

PDF exports were noticeably slower than image ZIP downloads under constrained network conditions because the response transferred a larger raw PDF payload. This change shifts PDF delivery to a compressed ZIP package while preserving generated PDF content.

• Backend response contract (PDF path)

  • Updated pdf_save=true result handling to return application/zip instead of raw application/pdf.
  • Response header now serves images.zip as attachment; archive contains images.pdf.

• PDF packaging utility

  • Added build_pdf_zip_bytes in backend/pdf.py to package PDF bytes into a ZIP in-memory stream (deflate compression).

• Targeted regression coverage

  • Added focused test to verify ZIP signature, expected entry name (images.pdf), and exact PDF byte integrity.
  • Added shared test import setup in backend test conftest.py.

with open(temp_pdf_file_path, "rb") as f:
    pdf_data = f.read()
zip_data = build_pdf_zip_bytes(pdf_data)
return Response(
    content=zip_data,
    media_type="application/zip",
    headers={"Content-Disposition": "attachment; filename=images.zip"},
)