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/http/retired-routes.ts

@@ -160,6 +160,14 @@ export const RETIRED_ROUTES: readonly RetiredRoute[] = [
       "Research Mode was an opt-in for the estimated drug-level curve. The chart had stopped consulting the flag several releases earlier and painted for every account regardless, so the switch governed nothing. The curve is simply part of the medication page now and needs no preference read.",
     methods: ["GET", "POST", "DELETE"],
   },
+  {
+    path: "/api/settings/preview-features",
+    removedIn: "1.46.0",
+    replacedBy: null,
+    reason:
+      "Stored per-account opt-ins to preview features, of which there was one, the topic overview pages. Those pages were dropped before they shipped, so the switch governed nothing; the route and its column are gone, and no preview is gated per account.",
+    methods: ["GET", "PATCH"],
+  },
 ];
 
 const BY_PATH: ReadonlyMap<string, RetiredRoute> = new Map(