Get Procurement Document Version Url

Mint a signed URL for one specific PAST version of a PO PDF.

Regenerate PDF (2026-08-05) now writes each generation to a distinct storage object (see _generate_pdf_for_items in manual_materials.py) so old versions' bytes survive later regenerates instead of being destructively overwritten — but the Track panel's doc_regenerated Timeline entries only ever had the CURRENT document's signed URL to link to. This resolves a signed URL for the specific storage_path recorded on that version's own timeline event, so clicking "v2" while "v4" is current actually opens v2's content, not v4's (Trevor: "all the versions are the same, all attachments have same po pdf").

storage_path is restricted to this project's own procurement-documents/ prefix — the caller only supplies the tail of a path already stamped on one of THIS project's own state_history entries, but this still blocks a crafted path from reaching into another project's folder.