Import common document attachments as PDFs (convert Office docs)

When an email has an attachment that's a common document type — Word (.doc/.docx), Excel (.xls/.xlsx), PowerPoint (.ppt/.pptx), and similar — let the user import it into the project as a redactable PDF.

We already support importing PDF attachments directly into a project. The gap is the non-PDF document types: they'd need to be converted to PDF on import (a conversion step we don't have yet) before they can enter the redaction workflow.

Goal: bring more attachment content into redaction — a user shouldn't have to download a Word/Excel attachment, convert it elsewhere, and re-upload it. Pick it from the attachment, we convert + import it as a PDF document in the project.

Notes / scope:

• Reuse the existing 'import attachment as document' flow; add a convert-to-PDF stage for supported non-PDF types.
• Decide the supported set (Office formats first; maybe images, .rtf, .txt, .eml later).
• Conversion likely belongs on the worker / a conversion service (LibreOffice headless or similar) — needs design.