[ADD] dms_libreoffice_preview: server-side office→PDF preview handler

[dnplkndll]

New sibling addon dms_libreoffice_preview — server-side LibreOffice→PDF conversion that plugs into the dms.preview_handlers registry introduced in #3. Fork-only; will be split as a standalone OCA submission once the base dms 19.0.1.0.0 wheel ships.

Base: 19.0-ci-ui-artifacts (#7) · Head: 19.0-add-dms-libreoffice-preview.

Stack: #3 UX → #9 Hoot → #7 CI → this PR.

What the branch adds

• dms_libreoffice_preview/ — new addon, depends: ["dms"], external_dependencies.deb: ["libreoffice", "fonts-noto", "fonts-liberation"] (OCA CI installs at runtime).
• Conversion: subprocess.run(["soffice", "--headless", "--convert-to", "pdf", ...]) per-conversion temp dir to avoid .~lock collisions.
• Cache: child ir.attachment on dms.file keyed by checksum (auto-cascades on file delete; ACL inherits via parent's check_access).
• Controller: /dms/file/<id>/libreoffice_preview — not readonly=True (writes the cached attachment), uses raise request.not_found() from e for ACL/missing.
• Frontend handler: LibreofficePreview registered at score 10 (beats built-in OfficeFallbackPreview at 0); renders an <iframe> reusing the existing .o_dms_preview__iframe SCSS class. text/rtf included in OFFICE_MIMETYPES so libmagic's text/rtf reply routes here instead of TextPreview.
• Tests: Python HttpCase covers happy path, cache hit, invalidation on checksum change, malformed file, ACL denial. Hoot covers registry score + URL composition.

Reviewer notes

• Subprocess per call is intentional (right for on-demand previews). unoserver daemon mode is a future optimisation.
• ACL enforced server-side via dms.file.check_access before serving the cached attachment.
• Cache key is checksum (content-addressed), not write_date — survives write_date bumps from unrelated field writes.

CI

GitHub Actions on the ledoent org is currently throttled. Runboat builds + local --test-enable pass.