Sync: upsertApiCommunicator called per cascade level — coalesce to a single push per sync

[RodriSanchez1]

Part of #2195.

Problem

updateApiObjectsNoChild (src/components/Board/Board.actions.js:1034) unconditionally calls upsertApiCommunicator(comm) after every board create. Because the function is also called recursively from updateApiMarkedBoards (:1086), a single user action that triggers a deep cascade (e.g. creating a board with N local children referenced by N parent boards) can produce N+1 PUT requests to the communicator endpoint within one sync cycle.

The doc already notes this in §14: "Communicator always persisted".

Impact

• Unnecessary load on the API.
• N times the failure surface (any of those PUTs failing causes the issues in Sync: surface partial failures via syncError instead of silent SYNC_BOARDS_SUCCESS #2196 / Sync: upsertApiCommunicator failures leave server-side communicator desynced silently #2197).
• Slower sync, particularly on flaky networks.

Proposed direction

• Coalesce: mark the communicator dirty during the cascade, push once at the end of pushLocalChangesToApi.
• Naturally combines with Sync: upsertApiCommunicator failures leave server-side communicator desynced silently #2197 if a communicator-level syncMeta is introduced — the dedicated drain step pushes once per sync.

Acceptance criteria

• A cascade that creates K boards results in at most one upsertApiCommunicator call per affected communicator per sync cycle.
• No regression in cross-device communicator consistency.