The PO history page previously allowed only a single status filter. This
enhances it to accept multiple statuses that are OR-ed together (e.g.
Closed + Approved shows all POs in either state), as requested.
- Status filter is now a multi-select checkbox dropdown that serialises
selections as repeated `status` query params.
- History page and the reports export endpoint read all `status` values
and query with `status: { in: [...] }` (OR semantics).
- Single-status and no-status cases remain unchanged.
Verified OR-query semantics against the test DB and confirmed both routes
compile and respond. type-check passes for the changed files.
Fixes#31
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
