David noticed: visible_at is a product timestamp, not a database timestamp

Kerra syncs content from course sites: announcements, assignments, quizzes, files, topics, and calendar events. each item has multiple possible times: when it was created in the database, when the lecturer created it, when it was modified, and when it became visible to students.

for simulation, David pushed toward a computed visibility timestamp. the simulated world should include only content that would have been visible at that simulated moment, not content merely ingested by the database.

that is why the distinction around visible_at mattered. it was not a naming nit. it defined whether simulated change detection leaks future course content into the past.