POST kicks off a calc-obs compute job: creates a CaseRun, enqueues a
Cloud Task targeting calc-obs, returns 201. Returns 409 + the existing
run if there's already an active (queued/running) run on the case so
the SPA can safely retry the "Run" button without spawning duplicates.

GET endpoints are pollable by the SPA. All write-fields on CaseRun are
server-managed via the calc-obs callback at /api/internal/runs/:id/update.

Required permissions:

• IsAuthenticated
• WorkspaceRequired
• HasRolePermissions::case_edit