Browser-triggered subscription sync.

The dashboard calls this from Paddle.js's checkout.completed callback so the user sees their new tier instantly — no need to wait for the webhook to round-trip through Paddle's servers. We re-fetch the canonical subscription from Paddle's API (don't trust the browser payload alone), then route through the same handler the webhook uses. Idempotency at the handler layer means a later webhook for the same subscription is a no-op.

Accepts { "subscription_id": "sub_..." } or { "transaction_id": "txn_..." }.