Add state-transition methods to ActivityRepository for advancing activity
stages (mark_downloaded, mark_converted, mark_imported, mark_duplicate,
mark_failed) with proper retention of last_completed_stage on failure.
Add list_pending_for_user to filter activities for processing. Implement
SyncRunRepository for creating and finalizing sync runs with counts and
summary errors.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
