[18.0] [IMP] spreadsheet_oca: email digest subscriptions for spreadsheets

[dnplkndll]

Adds email digest subscriptions: send a spreadsheet's live pivot summary to a
partner on a schedule.

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

Non-mechanical points worth flagging

• Each digest is computed as its own Run As user (with_user). This one
  matters more than elsewhere in the series because the result is emailed: a
  shared cron alone would compute every digest with the install user's rights
  and send it out.
• The mail carries an explicit email_from. Without one, delivery depends
  on mail.catchall.domain / mail.default.from being configured, and
  mail.mail logs the failure rather than raising it — so digests would go
  silently missing on a stock server.
• Pivot data is recomputed at send time, not read from the last browser save.

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