fix(relations): Fix duplicate messages in thread pagination

The /relations endpoint incorrectly handled pagination tokens, causing
duplicate messages when scrolling through threads. This was due to:

1. Incorrectly modifying the 'from' parameter with saturating_inc()
2. Not excluding the 'from' event from results

Changed pagination to use ShortEventId directly as the next_batch token
instead of complex PduCount handling. This provides more reliable
pagination that handles federation edge cases.

The fix ensures proper event ordering and prevents duplicates by:
- Using ShortEventId (u64) directly in pagination tokens
- Properly filtering events based on direction
- Excluding the starting event from results

src/api/client/relations.rs

@@ -109,13 +109,13 @@ async fn paginate_relations_with_filter(
 	recurse: bool,
 	dir: Direction,
 ) -> Result<get_relating_events::v1::Response> {
-	let start: PduCount = from
-		.map(str::parse)
-		.transpose()?
-		.unwrap_or_else(|| match dir {
+	let start: PduCount = from.and_then(|s| s.parse::<u64>().ok()).map_or_else(
+		|| match dir {
 			| Direction::Forward => PduCount::min(),
 			| Direction::Backward => PduCount::max(),
-		});
+		},
+		PduCount::Normal,
+	);
 
 	let to: Option<PduCount> = to.map(str::parse).flat_ok();
 
@@ -156,9 +156,7 @@ async fn paginate_relations_with_filter(
 		| Direction::Forward => events.last(),
 		| Direction::Backward => events.first(),
 	}
-	.map(at!(0))
-	.as_ref()
-	.map(ToString::to_string);
+	.map(|(count, _)| count.into_unsigned().to_string());
 
 	Ok(get_relating_events::v1::Response {
 		next_batch,

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();