[18.0] [IMP] spreadsheet_oca: writeback with audit log and rollback

[dnplkndll]

Adds writeback: edit a List cell in a spreadsheet and update the underlying Odoo
record, with an audit log and rollback.

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

Non-mechanical points worth flagging

• Every write runs as the user, checked before it happens. The controller
  verifies writeback is enabled, the spreadsheet is readable, the model is
  registered, the record exists, check_access("write") passes on that record,
  and the field exists and is neither readonly nor computed.
• Writeback is restricted to simple value fields (WRITEBACK_FIELD_TYPES).
  The previous value is stored as text so it can be rolled back, and a
  relational value does not survive that round-trip — str(recordset) yields
  res.partner(7,), which cannot be written back.
• Rollback converts before writing. Booleans are the reason: str(False)
  is "False", and a non-empty string is truthy, so writing it straight back
  would set the field to True.
• The audit log itself is written with sudo() so a permitted write is always
  recorded, regardless of the user's rights on the log model.

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