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/collab/lease.go

@@ -195,3 +195,23 @@ func (s leaseSnapshot) HolderOf(blockID string) (string, bool) {
 	c, ok := s[blockID]
 	return c, ok
 }
+
+// Sweep drops expired leases and sessions of all documents and forgets documents nobody uses any more,
+// so the maps do not grow with every note ever opened.
+func (m *LeaseManager) Sweep() {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	now := m.now()
+	for docID := range m.leases {
+		m.prune(docID, now)
+		if len(m.leases[docID]) == 0 {
+			delete(m.leases, docID)
+		}
+	}
+	for docID := range m.sessions {
+		m.prune(docID, now)
+		if len(m.sessions[docID]) == 0 {
+			delete(m.sessions, docID)
+		}
+	}
+}