feat(models): admin drag-and-drop sort order for the model picker

[philmerrell]

Why

The chat model picker shows models newest-first, so its order depends on when each model happened to be added. Admins had no way to put the models they want people to choose at the top.

What changed

Backend

• ManagedModel has a new read-only field, sortOrder (optional int). Only the new endpoint writes it. Create and update never touch it, so saving the model form can't undo a drag.
• New endpoint PUT /admin/managed-models/order takes { "modelIds": [...] }, the full list of model IDs in order, and sets each model's position.
  • It returns 409 if the IDs repeat or don't match the current table exactly. That means the admin reordered a stale copy, for example after another tab added or deleted a model.
  • It checks against a fresh table scan, not the 60s config cache.
  • It only writes rows whose position changed. Each write is conditional on the row still existing, so a model deleted mid-reorder isn't recreated as a bare row.
  • It clears the cache even if it fails partway.
  • It's declared before PUT /managed-models/{model_id}, so order isn't treated as a model ID. A test covers this.
• Sorting happens in _list_managed_models_cloud: ordered models first, lowest number first, then unordered models newest first. /models and the admin list therefore return the same order, and the picker keeps rendering what the server sends.
  • No migration. With nothing ordered, the list comes back exactly as it does today.
  • A newly added model goes to the bottom until an admin places it.

Frontend (Manage Models page)

• Each row has a CDK drag handle (cdkDropList / cdkDrag, locked to the vertical axis).
• Keyboard reordering on the handle: arrow keys move one place, Home/End move to the top or bottom. Focus returns to the handle after each move, and a live region announces each move to screen readers.
• Reordering is off while a search or filter is active, because dropping a row into a filtered subset has no clear meaning for the hidden models. The page shows "Clear filters to reorder" instead.
• ManagedModelsService.reorderModels updates the list immediately. Saves go out one at a time, and moves made while a save is in flight are merged into one follow-up save of the latest order, so a late response can't overwrite a newer local order. If a save fails, the list reloads from the server.
• Which model new chats start with is still set by isDefault. It doesn't depend on list position, so reordering doesn't change anyone's default model.

Testing

• Backend: 7 new storage tests (moto) and 4 new route tests. tests/routes, tests/shared and tests/architecture pass.
  • Locally, 8 inference/quota route tests errored because a dev .env was copied into the worktree to run a local server. They pass 40/40 with that file moved aside. It's a local-environment issue, not caused by this PR.
• Frontend: 3 new ManagedModelsService specs (optimistic update, coalescing, reload on failure). The manage-models, model picker (model-dropdown) and model-service specs pass (104). ng build is clean.
• ⚠️ Not yet checked by hand in the browser. The local sign-in needed Entra credentials. Worth one drag and one keyboard move on dev after merge.

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