Calculates unique maintainers count for a section based on EventLog.

Maintainers are users who have successfully created or updated:

• The section itself (direct changes)
• Notes belonging to this section (via metadata.section_id)

Uses raw SQL for performance with large datasets: the database counts, one statement for the whole batch of sections. A section with a site_id counts only the events of its own site; a section with none (a master section) counts the events of every site. A section with no events counts 0.