On a saved-filter (or view-filter) kanban view, checkBucketLimit counted
the total number of tasks matching the filter instead of the number of
tasks actually in the target bucket. Adding the first task to an empty
limited bucket was therefore wrongly rejected with code 10004
"exceeded the limit", even though the bucket was at 0/limit. The same
setup on a regular project bucket worked because that branch counts
task_buckets rows scoped to the bucket.
Scope the count to the bucket by adding `bucket_id = <id>` to the
TaskCollection filter. ReadAll combines this with the saved-filter /
view filter, so the count reflects exactly the tasks that are in this
bucket and match the filter. This keeps the #355 behaviour (stale
task_buckets rows whose tasks no longer match the filter are excluded)
while fixing the unscoped over-count.
Fixes#2672
cf456fb223