ensureUserWorkspace in auth.go omitted the IssuePrefix field when
creating a default workspace during first login. This caused the
workspace's issue_prefix to be empty, producing identifiers like "-16"
instead of "JIA-16".
- Pass generateIssuePrefix(wsName) when creating the default workspace
- Add fallback in getIssuePrefix to regenerate from workspace name if
the stored prefix is empty
- Add migration 024 to backfill empty prefixes on existing workspaces
