The counter wasn't being properly reset after renumbering because
SyncAllCounters uses MAX(old, new) which kept higher values from
deleted issues.
Solution: Add ResetCounter() method to delete the counter entry,
then SyncAllCounters recreates it from the actual max ID in database.
Now after renumbering 108 issues to bd-1..bd-108, the counter is
correctly set to 108 and next issue will be bd-109.