fix(tickets): fix statuses endpoint, members auth gate, and graceful error handling

- Add GET /boards/{board_id}/statuses endpoint — direct board-to-statuses lookup
  without ticket roundabout; used by filter bar and new ticket form
- Fix TicketsPage and NewTicketModal to call getBoardStatuses(board_id) instead
  of misusing getTicketStatuses(ticket_id) with a board_id value
- Fix list_members auth: was require_account_owner (owner/super_admin only) —
  changed to require_engineer_or_admin so engineers can see member list for
  ticket assignment
- list_members: return [] on PSAError instead of 502 (Lesson 111 pattern)
- get_ticket_statuses: return [] on PSAError instead of 502

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/tickets/NewTicketModal.tsx

@@ -44,7 +44,7 @@ export function NewTicketModal({ defaultTab = 'quick', initialValues, summaryHin
 
   useEffect(() => {
     if (draft.board_id) {
-      integrationsApi.getTicketStatuses(String(draft.board_id))
+      integrationsApi.getBoardStatuses(draft.board_id)
         .then(setStatuses).catch(() => {})
     } else {
       setStatuses([])