[18.0] [IMP] spreadsheet_oca: headless XLSX export with server-side pivot rendering

[dnplkndll]

Adds headless XLSX export: generate a workbook server-side, with no browser
session, addressing the gap in odoo/o-spreadsheet#8061.

Depends on #99 — its two commits are shared and drop out once
that merges.

Non-mechanical points worth flagging

• Pivot sheets are recomputed at export time rather than dumping the values
  last saved from the browser, so an export is never stale.
• Ids are used as keys, labels are what get written. Group matching needs
  the id-based rowValues; every cell written and every sort uses
  rowLabels, or a many2one column exports as a database id.
• Uses openpyxl, which ships in Odoo's own requirements.txt, so no
  external_dependencies entry is required.

AI-assisted (Claude Code); every change reviewed, tested, and owned by the author.