refactor(settings): remove the preview-feature opt-ins

The per-account preview switch was kept for the topic overview pages,
which were dropped before they shipped. Its one flag was stored, served
and backed up, and read by nothing; the column-reader audit listed it as
a pending consumer that will not come.

Removed completely:

- `/api/settings/preview-features` (GET, PATCH) is retired: it answers
  410 Gone with `meta.errorCode: route.retired` and `removedIn: 1.46.0`,
  and the contract publishes it under Retired. The route, its validation
  schema, the `PreviewFeatures` OpenAPI schemas and the settings module
  are deleted.
- Migration 0485 drops `users.preview_features_json` under a bounded lock
  wait, with the retry marker.
- The backup no longer writes the field. A file from v1.44 or v1.45 that
  carries it still restores: the schema accepts it and no codec reads it,
  so nothing is refused or written for it. The backup-parity goldens
  lose the field and apply one setting fewer.
- The data-wipe plan entry and the sharing-surface actor entry go with
  the route (frozen count 273 -> 272).

No client build reads the route: neither iOS repository nor any local
checkout references it.

src/lib/validations/settings-prefs.ts

@@ -1,8 +1,8 @@
 /**
  * Request schemas for the per-user Settings preferences that are not part of
  * the account payload: the list presentation of the labs, illness and
- * checkups lists, and the preview-feature opt-ins. Shared by the routes and
- * the OpenAPI registry so the wire contract has one source.
+ * checkups lists. Shared by the route and the OpenAPI registry so the wire
+ * contract has one source.
  */
 import { z } from "zod/v4";
 
@@ -26,12 +26,3 @@ export const moduleListLayoutPutSchema = z
     sortDir: z.enum(MODULE_SORT_DIRS).optional(),
   })
   .strict();
-
-export const previewFeaturesPatchSchema = z
-  .object({
-    topics: z.boolean().optional(),
-  })
-  .strict()
-  .refine((body) => Object.keys(body).length > 0, {
-    message: "Name at least one preview feature.",
-  });