fix(relations): improve thread pagination and include root event

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.

src/service/rooms/pdu_metadata/data.rs

@@ -61,9 +61,10 @@ impl Data {
 		from: PduCount,
 		dir: Direction,
 	) -> impl Stream<Item = (PduCount, impl Event)> + Send + '_ {
+		let from_unsigned = from.into_unsigned();
 		let mut current = ArrayVec::<u8, 16>::new();
 		current.extend(target.to_be_bytes());
-		current.extend(from.saturating_inc(dir).into_unsigned().to_be_bytes());
+		current.extend(from_unsigned.to_be_bytes());
 		let current = current.as_slice();
 		match dir {
 			| Direction::Forward => self.tofrom_relation.raw_keys_from(current).boxed(),
@@ -73,6 +74,17 @@ impl Data {
 		.ready_take_while(move |key| key.starts_with(&target.to_be_bytes()))
 		.map(|to_from| u64_from_u8(&to_from[8..16]))
 		.map(PduCount::from_unsigned)
+		.ready_filter(move |count| {
+			if from == PduCount::min() || from == PduCount::max() {
+				true
+			} else {
+				let count_unsigned = count.into_unsigned();
+				match dir {
+					| Direction::Forward => count_unsigned > from_unsigned,
+					| Direction::Backward => count_unsigned < from_unsigned,
+				}
+			}
+		})
 		.wide_filter_map(move |shorteventid| async move {
 			let pdu_id: RawPduId = PduId { shortroomid, shorteventid }.into();