Sync Revenuecat Id

Save the RevenueCat user ID on the current user's profile.

Called by the mobile app after login or first purchase so that webhook events can be correlated back to the Earmark user.

IDEMPOTENT, AND SAFE IN THE FIRST SECONDS OF AN ACCOUNT The app calls this on every open and every login (20 Sep 2026: a 500 eight seconds after sign-up). So:

• the same id again writes nothing and answers 200;
• no id (no body, null, blank) is a 200 no-op;
• a replay that fails — even one that leaves the session in a failed transaction — cannot turn the answer into a 500. The id is already committed by then; the session is rolled back so the request's closing commit is clean.