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/storage/sqlite_test.go

@@ -574,7 +574,7 @@ func TestDatabaseFirstWorkspaceArchitecture(t *testing.T) {
 		}
 	}
 
-	_, err = store.CreateDocShare(userA.ID, doc.ID, userB.ID, "write")
+	_, err = store.CreateDocShare(userA.ID, doc.ID, userB.ID, "read")
 	if err != nil {
 		t.Fatalf("CreateDocShare failed: %v", err)
 	}