core: deliver a store purchase to the profile it was bought under; verify store receipts safely

docs/protocol/badges-rpc.md

@@ -32,7 +32,7 @@ No command lets the client state what is signed. The tier is that of whatever fu
 - `getBadgeCatalog` → `badgeCatalog` — the prices and offers; signed, also the purchase's `badgeStatement`. Store builds never send it: prices come from the store and SKUs from app config.
 - `getBadgeInvoice` → `badgeInvoice` — prices the purchase for `badgeInfo` and `paymentVia` (`card` — Stripe; `crypto` — btc, xmr). The response holds the generic `invoice` — `invoiceId`, `price`, `discount`, the upgrade `credit`, `amount` = price − discount − credit, `currency`, `expiresAt`, and `paymentTo` (`url` for card; `address` and `cryptoAmount` for crypto) — beside the badge part, `badgeType` and `months`. `priceId` pins the price the client displayed; `offerId` selects a discounted duration, and its absence buys one month at that price. Price and offer status is checked here only: `deprecated` is still accepted, `disabled` is rejected; a badge type with no active price yields `product_unavailable`.
 - `redeemBadgeCode` → `badgeCredential` — redeems a code, records the credit, and issues the first credential, in one round trip. It carries `masterKey` and `code` and no `badgeRequest`: a code states no tier and no expiry, so the credential is what reports them. Errors: `code_invalid` for an unknown or malformed code, `code_used` when another key redeemed it, `code_expired` past a redemption deadline.
-- `purchaseBadge` → `badgeCredential` — verifies the funding (`apple` JWS offline; `google` product id and token via the Publisher API; `invoice` against webhook-confirmed settlement, `payment_pending` until it lands; `receipt`), records the credit, and issues the first credential, in one round trip. It carries `masterKey` and no `badgeRequest`: the tier and months are those of the product the funding proves, and the expiry is the one every credential for that week shares, so the client has nothing to state. Errors: `receipt_invalid` for a receipt the store does not vouch for, whether forged, malformed, unknown or refunded; `receipt_used` when another key was credited with it; `product_unavailable` for a product that grants no badge; `payment_pending` while the store has not settled it and `provider_unavailable` while the store cannot be asked, both recording nothing. The response `receipt` is the recovery bearer secret (model § recovery); the service stores its hash; lifetime badges receive none.
+- `purchaseBadge` → `badgeCredential` — verifies the funding (`apple` JWS offline; `google` product id and token via the Publisher API; `invoice` against webhook-confirmed settlement, `payment_pending` until it lands; `receipt`), records the credit, and issues the first credential, in one round trip. It carries `masterKey` and no `badgeRequest`: the tier and months are those of the product the funding proves, and the expiry is the one every credential for that week shares, so the client has nothing to state. Errors: `receipt_invalid` for a receipt the store does not vouch for, whether forged, malformed, unknown or refunded, and for a test purchase, which cost nothing; `receipt_used` when another key was credited with it; `product_unavailable` for a product that grants no badge; `payment_pending` while the store has not settled it and `provider_unavailable` while the store cannot be asked, both recording nothing. A receipt already credited to the signing key is answered from the service's record without asking the store. The client does not finish the store transaction on `product_unavailable`: the purchase is paid, and a product the service does not price is its operator's error, so the transaction is presented again at the next trigger rather than retried now. The response `receipt` is the recovery bearer secret (model § recovery); the service stores its hash; lifetime badges receive none.
   - Funding by `receipt` is a transfer (post-MVP): the unissued months of the purchase that receipt belongs to move to the signing key, recorded as `debit(transferOut)` on the source and `credit(transferIn)` on the new purchase, and the presented receipt is retired for a fresh one. The transferred period's issuance debits a month like any other. Lifetime badges hold no receipt, so support handles them.
 - `upgradeBadgeSubscription` → `badgeCredential` — the app-led store subscription change, on the same key: verifies the store evidence of the replaced subscription and records the new plan; an immediate upgrade returns the new credential, a deferred change returns none. Its `badgeRequest` is to be dropped (see above).
 - `issueBadge` → `badgeCredential` — issues the next period from the balance, the only source of issuance. It carries `balance` alone: the credential is signed with the purchase's stored master key, for the type the balance funds, expiring at the `sundayAfter` of the period issued. The ledger is advanced first; the credential is signed before the `debit(badge)` and issuance rows are written, in one transaction. An exhausted balance yields no `credential`; the `statement` shows why. Issuing on a paused badge resumes it (model 2.13).
@@ -63,4 +63,4 @@ An assertion that names an entry the service holds is a prefix: the service proc
 
 ## Errors
 
-`retryAfter` marks the transient codes: `payment_pending`, `provider_unavailable`, `rate_limited`. `offer_disabled` calls for a catalog refresh. `code_invalid` covers unknown, malformed and revoked codes alike, so a guesser learns nothing from the difference; `code_used` — redeemed under another key; `code_expired` — past its redemption deadline. `receipt_invalid` covers unknown receipts. All other codes are terminal for the attempted command.
+`retryAfter` marks the transient codes: `payment_pending`, `provider_unavailable`, `rate_limited`. `offer_disabled` calls for a catalog refresh. `code_invalid` covers unknown, malformed and revoked codes alike, so a guesser learns nothing from the difference; `code_used` — redeemed under another key; `code_expired` — past its redemption deadline. `receipt_invalid` covers forged, malformed, unknown, refunded and test receipts alike, so a guesser learns nothing from the difference; `receipt_used` — credited to another key. All other codes are terminal for the attempted command.