POST/GET /api/assignments mirror the existing entity route pattern
(auth-gated, thin handler delegating to src/lib/assignments.ts). PATCH
/api/assignments/[id] is the first dynamic-segment route in the codebase,
per the stakeholder-confirmed convention (docs/asset-license-management.md)
of update routes being PATCH /api/<entity>/[id] rather than an
action-based sub-route.
