Shopify Credits Sync

Ensure the shop's headless user + wallet, apply the plan's credit grant, and return the current balance. The Shopify app calls this on every load (cheap). Grant shape depends on the plan:

• free: a ONE-TIME signup grant into the non-expiring bonus pool (add_studio_bonus_credits, action_type shopify_signup_free) — fired only if this shop has never received a free grant (neither the signup grant nor a legacy shopify_free_monthly from the pre-billing era). Never refilled: once spent, the merchant upgrades or stops.
• starter: a monthly lazy reset of the subscription studio_credits pool via grant_studio_credits (SET + expiry on the 1st) — fires when the current grant has lapsed, so mid-month consumption is preserved. On upgrade the app passes force=true once so the paid allotment takes effect immediately; downgrades don't force, so a paid month's balance survives until the 1st, after which the free plan grants nothing further.

A Shopify shop account never has a Stripe studio subscription (synthetic email, no login), so the subscription pool isn't contended. Idempotent on email (like provision-user). X-Internal-Key gated.