fix(handler): set issue_prefix when auto-creating workspace on first login

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

server/migrations/024_backfill_empty_issue_prefix.down.sql

@@ -0,0 +1 @@
+-- No-op: we cannot reliably determine which workspaces previously had empty prefixes.