Returns the mobile (App Store) subscription entitlement stored on the authenticated user. `isActive` is true only when the stored entitlement is active and its `expiresAt` is unset or in the future. When the entitlement is not active and `expiresAt` has passed, `status` is reported as `expired`; otherwise the stored status is returned (`none` for users who never validated a purchase). No call to Apple is made; the data reflects the last validation.

Validates proof of an App Store subscription purchase with Apple and saves the resulting entitlement on the authenticated user. At least one of `transactionReceipt`, `transactionId` or `originalTransactionId` must be sent. When the server is configured for the App Store Server API and a transaction id is supplied, the transaction and its subscription status are fetched from Apple (Production first, then Sandbox); otherwise the `transactionReceipt` is verified through Apple's receipt verification endpoint. The purchase must belong to the Sharh app bundle and to a recognised subscription product (by default `sharh.monthly.subscription` or `sharh.yearly.subscription`). The latest transaction for a recognised product determines the result. An App Store subscription can be linked to only one Sharh account: if another user already holds the same `originalTransactionId`, the request fails with 409. On success the stored entitlement is overwritten and returned in the same shape as `GET /iap/subscription`. `status` is `revoked`, `none` (no matching subscription found), Apple's numeric subscription status as a string (for example `"1"` active, `"4"` billing grace period) on the Server API path, or `active` / `expired` on the receipt path. `source` is `app_store_server_api` or `verify_receipt`. A validation that finds an inactive or expired subscription still returns 200 with `isActive: false`.