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/openapi/routes/settings.ts

@@ -27,10 +27,7 @@ import {
   MODULE_LIST_VIEWS,
   MODULE_SORT_DIRS,
 } from "@/lib/settings/module-list-layout";
-import {
-  moduleListLayoutPutSchema,
-  previewFeaturesPatchSchema,
-} from "@/lib/validations/settings-prefs";
+import { moduleListLayoutPutSchema } from "@/lib/validations/settings-prefs";
 
 const moduleListPrefsEntry = z
   .object({
@@ -72,20 +69,6 @@ const moduleListLayoutSchema = z
       "Per-user presentation of the labs, illness and checkups lists. Display-only; a client that does not render a list ignores it.",
   });
 
-const previewFeaturesSchema = z
-  .object({
-    topics: z
-      .boolean()
-      .describe(
-        "Topic overview pages above the single-metric Insights pages. Off by default.",
-      ),
-  })
-  .meta({
-    id: "PreviewFeatures",
-    description:
-      "Per-user opt-in to preview features, one boolean per flag, every flag off by default.",
-  });
-
 // v1.16.11 — the one threshold read every dose-status consumer makes
 // (cards, table, take-all-due derivation). `lateMinutes` /
 // `missedMinutes` are the operator-level singleton; the low-stock
@@ -970,57 +953,6 @@ export const settingsPaths: NonNullable<ZodOpenApiObject["paths"]> = {
       },
     },
   },
-  "/api/settings/preview-features": {
-    get: {
-      tags: ["Meta"],
-      summary: "Get the preview-feature opt-ins",
-      description: "Every flag, off unless the caller switched it on.",
-      responses: {
-        "200": {
-          description: "The opt-ins.",
-          content: {
-            "application/json": {
-              schema: dataEnvelope(
-                previewFeaturesSchema,
-                "PreviewFeaturesEnvelope",
-              ),
-            },
-          },
-        },
-        ...stdResponses,
-      },
-    },
-    patch: {
-      tags: ["Meta"],
-      summary: "Switch preview features on or off",
-      description:
-        "Sets the flags the body names and keeps the rest. An unknown flag or an empty body is refused with 422.",
-      requestBody: {
-        required: true,
-        content: {
-          "application/json": {
-            schema: previewFeaturesPatchSchema.meta({
-              id: "PreviewFeaturesPatchBody",
-            }),
-          },
-        },
-      },
-      responses: {
-        "200": {
-          description: "The opt-ins as stored.",
-          content: {
-            "application/json": {
-              schema: dataEnvelope(
-                previewFeaturesSchema,
-                "PreviewFeaturesSavedEnvelope",
-              ),
-            },
-          },
-        },
-        ...stdResponses,
-      },
-    },
-  },
   "/api/settings/privacy-summary": {
     get: {
       tags: ["Meta"],