Delete UserReports after retention

The UserReport postgres model is used for backwards compatibility w/older user feedback sources, and also to associate feedback to groups (issues). A user report is only valid if its associated `event_id` + `group_id` exists in the same project.

Therefore to save COGS, we can delete reports after the max event retention (90d(?)). Maybe double to 180d so we can have debugging data. As long as we're not keeping forever.

No files or tests are named. Start by locating the UserReport PostgreSQL model and the existing event-retention cleanup path, then establish whether the retention period should be 90 or 180 days. Done means reports are deleted after the chosen maximum event-retention period without retaining them indefinitely.

Written by the indexing model from the issue text.