List Pending Tree

Hierarchical pending-queue view (spec f33eb9ca — Layer 4 Pending Queue UI). Returns a 5-level tree: Ideations → Refinements → Specs → Sprints → Cards, each level annotated with aggregate status counters drawn from `consolidation_queue`. The UI renders this via `frontend/src/components/knowledge/PendingQueueTree.tsx` with lazy expansion by level (BR `Tree Lazy Fetch por Nível`). Payload shape (stable — consumed by React component): { "board_id": str, "total_pending": int, "levels": {ideations: {pending,in_progress,done,failed}, refinements: ..., specs: ..., sprints: ..., cards: ...}, "tree": [ideation-nodes with nested children] }