test(appview): add missing DB error 503 tests for board endpoints (ATB-45)
- POST /api/admin/boards: add "returns 503 when category lookup query fails"
- PUT /api/admin/boards/:id: add "returns 503 when board lookup query fails"
- PUT /api/admin/boards/:id: add "returns 503 when category CID lookup query fails"
(call-count pattern: first select passes, second throws)