Fitbit, WHOOP, and Oura all issue one-time-use refresh tokens. The token
persistence did an unguarded read → refresh → write, so two overlapping
syncs for one user both read the same stored refresh token and both POST
it. The provider honours only the first; the loser's refresh comes back 401,
gets classified as reauth, and the connection is parked at `error_reauth`
spuriously even though the peer rotated the token successfully.
Add a shared `persistRotatedToken` helper that writes the rotated pair with
a compare-and-swap on the stored ciphertext (matched verbatim, since AES-GCM
re-encryption of the same plaintext differs byte-for-byte). When the
conditional update touches zero rows a peer already won the race; re-read the
row and reuse the peer's freshly rotated access token instead of erroring.
Wire it into all three providers — Fitbit and WHOOP `getValidToken`
(proactive expiry refresh, keyed by connection id) and Oura `storeOuraTokens`
(reactive 401 refresh, keyed by user id). Both racing callers now end with a
usable token and neither drops the connection.
Marc-André Bombeck