perf: export_trial_v2 recomputed on every job request (admission-trial routes) #34

S2 (PR #33) serves the signed job route by calling export_trial_v2 on each request; the public island_job_v2 route has a similar cost. Decide caching or a shared builder for both job routes together. Not a blocker (option A, Lens 2026-10-25).