graph/db: convert ChanUpdatesInHorizon to use iterators

In this commit, we refactor the ChanUpdatesInHorizon method to return
an iterator instead of a slice. This change significantly reduces
memory usage when dealing with large result sets by allowing callers to
process items incrementally rather than loading everything into memory
at once.

graph/db/interfaces.go

@@ -220,8 +220,8 @@ type V1Store interface { //nolint:interfacebloat
 	// ChanUpdatesInHorizon returns all the known channel edges which have
 	// at least one edge that has an update timestamp within the specified
 	// horizon.
-	ChanUpdatesInHorizon(startTime, endTime time.Time) ([]ChannelEdge,
-		error)
+	ChanUpdatesInHorizon(startTime, endTime time.Time,
+		opts ...IteratorOption) (iter.Seq[ChannelEdge], error)
 
 	// FilterKnownChanIDs takes a set of channel IDs and return the subset
 	// of chan ID's that we don't know and are not known zombies of the