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/api/integrations.ts

@@ -30,6 +30,8 @@ export const integrationsApi = {
     apiClient.get<PSATicketInfo>(`/integrations/psa/tickets/${id}`).then(r => r.data),
   getTicketStatuses: (ticketId: string) =>
     apiClient.get<PSATicketStatusItem[]>(`/integrations/psa/tickets/${ticketId}/statuses`).then(r => r.data),
+  getBoardStatuses: (boardId: number | string) =>
+    apiClient.get<PSATicketStatusItem[]>(`/integrations/psa/boards/${boardId}/statuses`).then(r => r.data),
   listMembers: () =>
     apiClient.get<PsaMemberResponse[]>('/integrations/psa/members').then(r => r.data),
   getMemberMappings: () =>