v2.0.0

Panelist 2.0.0 makes rubric versions immutable. Publishing a new version through POST /rubrics/{id}/versions creates the next version, moves every untouched queued task onto it, and reports how many open tasks still reference the previous version. A task pins its rubric version at claim time, so an expert who was shown version 1 keeps grading version 1 even when version 2 is published during the lease, and the stored grade references the pinned version. A grade that names a superseded version is rejected with a 409 that states both versions, new tasks cannot target a superseded version, and only the current version can be published from. Criterion analytics group by rubric version and delivery rows carry the version each grade was produced against. The suite grows to 44 tests, all exercised through the HTTP API against PostgreSQL.