feat(notes): operation log, block leases and sync API for collaborative editing

Phase 1 of the collaboration plan (server side):
- internal/collab: block tree with parent IDs and fractional order keys,
  operations with conflict rules (version check, block leases, conflict
  copies instead of lost text, restore on late edits, cycle checks),
  in-memory leases/sessions and long-poll notifier.
- Storage: per-note lock + transaction, append-only effect log (doc_ops),
  idempotent results per opId (doc_op_results), snapshot, log paging,
  central DocRole. Old flat block data is converted to the tree.
- The existing whole-document save (PUT/PATCH) runs as an adapter through
  the same engine, so the current editor keeps working and writes to the log.
- API: GET /docs/{id}/snapshot, POST /docs/{id}/sync, GET /docs/{id}/changes
  (long poll), POST /docs/{id}/leave; DOC_SEQ poke for WebSocket/SSE rooms.
- Backend accepts editor (write) shares for notes and folders; the forced
  read-only migration is gone. The share dialog still offers read only.
- Tests: conflict matrix, order keys, leases, storage adapter round trip,
  idempotency, API protocol incl. long poll, and a randomized simulation
  (4 clients, lost/duplicated requests and responses, offline phases,
  lease expiry) checking convergence, no lost text and exactly-once results.

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

internal/collab/notify.go

@@ -0,0 +1,34 @@
+package collab
+
+import "sync"
+
+// Notifier wakes up long-poll requests waiting for changes of a document.
+type Notifier struct {
+	mu   sync.Mutex
+	wait map[string]chan struct{}
+}
+
+func NewNotifier() *Notifier {
+	return &Notifier{wait: map[string]chan struct{}{}}
+}
+
+// Changed returns a channel that is closed on the next Notify for docID.
+func (n *Notifier) Changed(docID string) <-chan struct{} {
+	n.mu.Lock()
+	defer n.mu.Unlock()
+	ch, ok := n.wait[docID]
+	if !ok {
+		ch = make(chan struct{})
+		n.wait[docID] = ch
+	}
+	return ch
+}
+
+func (n *Notifier) Notify(docID string) {
+	n.mu.Lock()
+	defer n.mu.Unlock()
+	if ch, ok := n.wait[docID]; ok {
+		close(ch)
+		delete(n.wait, docID)
+	}
+}