Remove admin model refresh feature
Reverts PR #1961 (Refresh models) and the follow-up commit c5cc4b8 that
added the admin "Refresh models" button. The feature added an admin-only
endpoint to rebuild the in-memory model registry on demand; we no longer
want it.
- Delete POST /api/v2/models/refresh endpoint
- Drop the matching client route in APIClient
- Remove the admin Refresh button from Application Settings
- Strip refresh state and helpers (refreshModels, lastModelRefresh*,
ModelsRefreshSummary, applyModelState, signatureForModel,
rebuildModels, inflightRefresh) from src/lib/server/models.ts;
models/defaultModel/taskModel/validModelIdSchema are now const,
built once at module load via buildModels()
