feat(12.1-01): declared bulk actions on admin lists

- pact.HasAdminBulkActions with AdminBulkAction, its input and result
- config_list.yaml bulkActions, compiled fail-loud, needs showCheckboxes
- POST .../{controller}/bulk/{action}: ids resolved and locked through the
  list scope in one transaction; partial selection is 409
- list schema offers declared actions per principal, with confirm text
- admin SPA bulk actions menu with confirm, busy state and failure toasts
- acme.roster fixture, tracer test, OpenAPI, TS types, dist, READMEs, docs

docs/backend/admin-spa.md

@@ -27,6 +27,8 @@ The SPA signs in through the admin API and keeps the token in the HttpOnly cooki
 
 Each record form makes a session key when it opens: 32 random bytes, base64url encoded. The form sends it in the `X-Session-Key` header with every file upload, file list and file removal, and with the final create or update save. Uploads and removals are deferred: the server holds them against the key and the admin, and the save that carries the same key commits them in its transaction. Until then the form counts as unsaved, so leaving it asks first, and a new record's files go to record id `0`. The form will not save while an upload is still in flight. Uploads use `XMLHttpRequest` for progress events and carry the same `X-Requested-With` header and cookie as every other call, plus an `X-Upload-Id` so a retry returns the already stored file. Files of a protected relation are fetched through the admin API with the key and shown from object URLs. The key travels only in headers, never in a URL. See [File uploads](forms.md#file-uploads) for the `fileupload` field.
 
+A list whose schema carries declared bulk actions shows a "Bulk actions" menu after the selection count. The menu lists only the actions the server offered to this administrator, and its button stays disabled until a row is selected. Choosing an action always asks for confirmation; the dialog stays open while the request runs, and the list reloads afterwards. See [Bulk actions](admin-controllers.md#bulk-actions).
+
 ## Types from OpenAPI
 
 The admin API is described by swag annotations in cabana. `scripts/check-admin-openapi.sh` generates the OpenAPI document (`admin/openapi/admin.json`) from them and the SPA's TypeScript types (`admin/src/api/schema.d.ts`) from the document, so the SPA's API client is checked against the server's shapes at compile time. `--check` fails when either committed file is out of date: