- Much simpler handling of downloads in the activity sidebar, and
improved storage, persistence and UI behavior.
- Replace `ActivityService` with direct storage on
`DownloadHistoryService` and `download_requests` and removes the
activity_log/activity_dismissals tables
- Simplify no-auth mode by removing the fake user row pattern, handled
internally
- Add local download fallback so history entries can still serve files
after tasks leave the queue
- Downloads, requests and history are now entirely persistent between
updates / restarts, and correctly tied to each user.
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