Replace unreliable PduCount pagination tokens with ShortEventId throughout
the relations and messages endpoints. ShortEventId provides stable, unique
identifiers that persist across server restarts and database operations.
Key improvements:
- Add token parsing helpers that try ShortEventId first, fall back to
PduCount for backwards compatibility
- Include thread root event when paginating backwards to thread start
- Fix off-by-one error in get_relations that was returning the starting
event in results
- Only return next_batch/prev_batch tokens when more events are available,
preventing clients from making unnecessary requests at thread boundaries
- Ensure consistent token format between /relations, /messages, and /sync
endpoints for interoperability
This fixes duplicate events when scrolling at thread boundaries and ensures
the thread root message is visible when viewing a thread, matching expected
client behaviour.
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