[18.0] [IMP] spreadsheet_oca: KPI alerts with cron-based cell monitoring

[dnplkndll]

Adds KPI alerts: watch a spreadsheet cell and notify when it crosses a threshold.

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

Non-mechanical points worth flagging

• trigger_mode is the design decision. Edge fires once when the
  condition first becomes true; level fires every cycle it holds. Without
  that distinction a cron would repost to Chatter every hour while a KPI sits
  below target.
• Each alert evaluates as its own Run As user (with_user), so an alert
  can only ever report on data its owner may read. The shared cron would
  otherwise evaluate everything as the install user.
• The evaluation state write is sudo(), deliberately. Owners need only
  read rights on the data; recording last_state / last_value is the
  module's own housekeeping, not a user edit. action_reset_state stays
  ACL-bound because that one is a user action.
• A cell that cannot be read logs a warning. Such an alert can never fire,
  and silence is indistinguishable from "threshold not crossed".

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