How an application resolves what a request should be billed to.
The escape hatch that keeps this module out of the business of guessing: an application knows
where its subject lives, whether that is a claim in a token, a field of a body, a header its own
edge signs, or a lookup a middleware of its own already did. Return nothing when the request
carries none, and never throw: a subject that cannot be read must degrade, not break the route.
How an application resolves what a request should be billed to.
The escape hatch that keeps this module out of the business of guessing: an application knows where its subject lives, whether that is a claim in a token, a field of a body, a header its own edge signs, or a lookup a middleware of its own already did. Return nothing when the request carries none, and never throw: a subject that cannot be read must degrade, not break the route.