feat(notes): compact the operation log and purge old tombstones

- Background job at start and every 6 h: log entries older than 30 days
  (private notes: 7) are removed, documents.log_floor marks the boundary;
  clients behind it get resync and keep their queued changes
- Deleted blocks are purged 30 days after deleted_at via a "purge" log
  effect, so open editors drop them without reloading; a tombstone stays
  while a deleted child still points to it
- Text for a purged block becomes a new block at the end; inserts under a
  purged parent go to the top level (server and browser model)
- doc_op_results kept for 90 days; expired lease/session maps are swept
- Tests: storage, API, engine resync, simulation with compactions

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/server.go

@@ -46,6 +46,7 @@ func NewServer(cfg *config.Config, store *storage.Store, wsHub *ws.Hub, staticFS
 	}
 	wsHub.SetRoomAuthorizer(srv.canJoinRoom)
 	go srv.startAlarmTicker()
+	go srv.startCompaction()
 	return srv
 }