feat(cabana): settings entries that link to an admin controller

pact.SettingsItem gains an additive Controller field, the equivalent of a
WinterCMS registerSettings 'url' => Backend::url(...) entry. A link entry
declares no Model, Form or NewModel and needs no AdminFS; start-up fails
when it combines Controller with a singleton form or a model, or names an
unregistered controller. Settings codes stay one namespace.

GET /settings lists a link entry with its controller only when the
principal passes the item's permissions and may open the controller.
Registry.Setting never returns a link entry, so the singleton settings
endpoints answer 404 for its code. SettingsEntry carries controller,
empty for singletons; the OpenAPI document, generated SPA types and
settings fixture follow, and the pact and cabana READMEs and the
settings docs describe the link.

modules/cabana/README.md

@@ -30,7 +30,7 @@ Schema-driven admin backend that compiles WinterCMS-style YAML list, form, filte
 - Date pickers: a `type: datepicker` field in `fields.yaml` edits a date (`mode: date`, a `lagoon.Date` column), a date and time (`mode: datetime`, the default, a `time.Time` column stored in UTC) or a time of day (`mode: time`, a `lagoon.TimeOfDay` column); pointers to the three types make the value optional. It accepts WinterCMS's `mode`, `format` (a PHP `date()` format, served also as `displayFormat` in the SPA's tokens), `minDate`, `maxDate`, `yearRange`, `firstDay`, `twelveHour` and `ignoreTimezone`; any other key, a format letter with no equivalent, bounds on `mode: time`, `ignoreTimezone` outside `mode: datetime` or a column whose Go type does not match the mode fails boot. The save rechecks `minDate` and `maxDate` on the calendar date and answers 422 on the field. List columns take `type: date` and `type: time` for these columns; when `type` is omitted, a `time.Time` column is compiled as `datetime`, a `lagoon.Date` column as `date` and a `lagoon.TimeOfDay` column as `time`. A struct column that implements `sql.Scanner` or `driver.Valuer` is never taken for a relation.
 - File uploads: a `type: fileupload` field in `fields.yaml` edits an attachOne or attachMany relation the record model declares through `attach.HasRelations` (its `AttachRelations` method) next to `attach.Owner`. The field accepts WinterCMS's `mode` (`image` or `file`), `fileTypes`, `mimeTypes`, `maxFilesize` (megabytes), `maxFiles` (attachMany only), `imageWidth`, `imageHeight`, `thumbOptions` (only `mode`: `auto`, `exact`, `crop` or `fit`), `useCaption` and `prompt`; any other key, an image-mode file type outside jpg, jpeg, png, gif and webp, a name that is not a declared relation or a `maxFilesize` whose file plus 64 KiB of multipart framing exceeds `http.body_limits.upload_bytes` fails boot. Uploads and removals are deferred, as in WinterCMS: the SPA sends a random form session key in the `X-Session-Key` header (`cabana.SessionKeyHeader`) with every file call and with the save, the server keeps the pending work in `deferred_bindings` against that key and the signed-in administrator, and the record's next create or update save applies it inside its transaction. A retry of the same upload may send `X-Upload-Id` so the server returns the already stored file. A save that fails with 422 keeps the pending uploads; another administrator's key matches nothing. The upload route caps the request body at the smaller of `http.body_limits.upload_bytes` and `maxFilesize` plus 64 KiB and answers 413 `payload_too_large` past it; the size, type and image checks run on the server (through `attach.Store`) and answer 422 on the field. A file list (`cabana.FileItem`) carries `url` and `thumb_url` only for a public relation.
 - Refusals: a lifecycle hook, a relation hook or a bulk, record, toolbar or widget action that returns a `cabana.ForbiddenError` is answered 403 `forbidden` with the error's `Message` and `Details` (a field name to a list of messages), both translated in the request locale; an empty `Message` stays empty. The surrounding transaction is rolled back. Every other error that is not a `cabana.ValidationError` stays the opaque 500, logged on the server.
-- Singleton settings screens declared with `pact.HasSettings`, read and saved by `cabana.SettingsService`.
+- Singleton settings screens declared with `pact.HasSettings`, read and saved by `cabana.SettingsService`. A settings entry whose `Controller` field is set links to that admin controller instead: start-up fails when it also declares a model, form or model factory, or names an unknown controller; `GET /settings` lists it, with its `controller`, only to administrators who pass its permissions and may open the controller; and the singleton settings endpoints answer 404 for its code.
 - Backend navigation (`pact.HasNavigation`) and permissions (`pact.HasPermissions`), filtered per user by `cabana.Registry.Metadata`. An administrator's own `backend_users.permissions` are merged over the role's as in Winter (a `-1` denies a code the role grants). `cabana.Allows` implements the permission check with Winter's `hasAnyAccess` semantics: superusers pass, a principal needs any one of the listed codes, and wildcards match on both sides (a grant ending in `.*` covers every code with that prefix, and a required code ending in `.*` is met by any grant under it).
 - Admin authentication against WinterCMS's `backend_users` and `backend_user_roles` tables (`cabana.BackendUser`, `cabana.BackendUserRole`, `cabana.BackendUsers`): a JWT guard registered in [bouncer](../bouncer/README.md) as `backend` (a guard another plugin already registered under that name fails `cabana.Activate`), login throttling, token refresh and revocation, and two transports. API clients use a Bearer token; the SPA sends `X-Requested-With: XMLHttpRequest` and receives the token in the HttpOnly, SameSite=Strict cookie named by `cabana.AdminCookieName`. Cookie-authenticated requests that change state must carry that header, which blocks cross-site request forgery.
 - A consistent JSON envelope for every response: `cabana.WriteData`, `cabana.WriteError` and `cabana.WriteErrorDetails`, typed for documentation as `cabana.Envelope`, `cabana.ListEnvelope`, `cabana.RecordEnvelope` and `cabana.ErrorEnvelope`. A body that cannot be encoded is logged and answered with the generic 500 envelope, never a success status with a truncated body.
@@ -46,7 +46,7 @@ All paths are relative to `<prefix>/api/v1`. A controller ID `vendor.plugin.cont
 | POST `/auth/login`, POST `/auth/refresh` | Sign in (throttled) and refresh a token. Public. |
 | GET `/lang` | The `backend::lang` string bundle for the request locale. Public, so the login screen can load it. |
 | POST `/auth/logout`, GET `/auth/me` | Revoke the current token, also when its access lifetime has expired but its refresh window is open, and clear the session cookie; return the signed-in administrator. |
-| GET `/navigation`, GET `/settings` | Navigation and settings entries the administrator may open. |
+| GET `/navigation`, GET `/settings` | Navigation and settings entries the administrator may open; a settings entry's `controller` names the admin controller a link entry opens and is empty for a singleton form. |
 | GET `/settings/{code}/schema`, GET and PUT `/settings/{code}` | Settings form schema, values and update. |
 | POST `/markdown/preview` | Render `{markdown}` through `cabana.RenderMarkdown` for the form preview and answer `{html}`; a refused output is a 422 `validation_failed` on `markdown`. Needs any backend session. |
 | GET `/{vendor}/{plugin}/{controller}/schema/list`, `.../schema/form`, `.../schema/relation/{name}` | Localized list, form and relation schemas. |
@@ -191,7 +191,7 @@ func (p *Plugin) AdminFS() fs.FS { return adminFS }
 | `cabana.CompileList` / `cabana.CompileForm` | Compile a controller's list and form YAML into cached schemas. |
 | `cabana.ListSchema` / `cabana.FormSchema` / `cabana.RelationSchema` | Locale-neutral compiled schemas; each request works on a localized copy. |
 | `cabana.CompiledController` | One controller after compilation: list, form, relations and writable fields. |
-| `cabana.Registry` | Immutable map of compiled controllers and settings, with permission-filtered metadata. |
+| `cabana.Registry` | Immutable map of compiled controllers and settings, with permission-filtered metadata. `cabana.Registry.Setting` returns singleton settings only, never a controller link. |
 | `cabana.CRUDService` | Schema-projected show, create, update, delete, bulk delete and relation options; `cabana.CRUDService.BulkAction` runs a declared bulk action on a scoped, locked id set. |
 | `cabana.BulkAction` | One entry of a list schema's `bulkActions`: name, localized label and optional confirm text. |
 | `cabana.BulkActionResult` | Answer of the bulk action route: the localized `message` and the `affected` count. |