Run up to 20 endpoint calls in one request

Runs up to 20 Captapi GET calls in one request, 10 at a time, through the same auth, credit, cache and rate-limit pipeline as direct calls. Each item is billed exactly like the direct call and failed items are not billed. Results keep input order; each one carries the item's HTTP status and its full response body. A failed item's body uses the standard error envelope {success: false, error: {code, message, retryable}}, including BATCH_ITEM_TIMEOUT and BATCH_ITEM_FAILED when the item could not finish. An unknown path fails the whole batch with 400 before anything runs.