X_BACKFILL_SOURCE_INVALID
Means
a backfill() declares both or neither of source and pages, or input with no scope
Typical cause
backfill() declared source and pages together, neither of them, or an input schema with no scope(input) to key one pass per scope
Fix
backfill({ name, tenant, pages, handle }) with exactly one of source or pages, and scope: (input) => input.accountId beside input