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/app/api/settings/preview-features/route.ts

@@ -1,80 +0,0 @@
-/**
- * GET / PATCH the caller's preview-feature opt-ins.
- *
- * GET answers every flag, off unless the caller switched it on. PATCH sets
- * the flags the body names and keeps the rest; unknown flags are refused
- * with the multi-issue 422 so a typo cannot silently do nothing. The blob
- * lives on its own `User` column (`preview_features_json`).
- *
- * The opt-ins are the caller's own, so this is an actor surface
- * (`requireActorAuth`): a browser acting for another record keeps reaching
- * the person's own switch. The PATCH reads and writes under a row lock, so two
- * flags switched at once both stay switched.
- */
-import { apiHandler, requireActorAuth } from "@/lib/api-handler";
-import { apiSuccess } from "@/lib/api-response";
-import { parseJsonBody } from "@/lib/api/body";
-import { prisma, toJson } from "@/lib/db";
-import { annotate } from "@/lib/logging/context";
-import {
-  PREVIEW_FEATURE_KEYS,
-  resolvePreviewFeatures,
-  type PreviewFeatures,
-} from "@/lib/settings/preview-features";
-import { previewFeaturesPatchSchema } from "@/lib/validations/settings-prefs";
-
-export const dynamic = "force-dynamic";
-
-async function readStored(
-  userId: string,
-  db: Pick<typeof prisma, "user"> = prisma,
-): Promise<unknown> {
-  const row = await db.user.findUnique({
-    where: { id: userId },
-    select: { previewFeaturesJson: true },
-  });
-  return row?.previewFeaturesJson;
-}
-
-function enabledFlags(features: PreviewFeatures): string[] {
-  return PREVIEW_FEATURE_KEYS.filter((key) => features[key]);
-}
-
-export const GET = apiHandler(async () => {
-  const { user } = await requireActorAuth();
-  const features = resolvePreviewFeatures(await readStored(user.id));
-  annotate({
-    action: { name: "settings.preview-features.read" },
-    meta: { enabled: enabledFlags(features) },
-  });
-  return apiSuccess(features);
-});
-
-export const PATCH = apiHandler(async (request: Request) => {
-  const { user } = await requireActorAuth();
-
-  const parsed = await parseJsonBody(request, previewFeaturesPatchSchema, {
-    maxBytes: 1024,
-    action: "settings.preview-features.validation-failed",
-  });
-  if (!parsed.ok) return parsed.error;
-
-  const next = await prisma.$transaction(async (tx) => {
-    await tx.$queryRaw`SELECT 1 FROM "users" WHERE "id" = ${user.id} FOR UPDATE`;
-    const current = resolvePreviewFeatures(await readStored(user.id, tx));
-    const merged: PreviewFeatures = {
-      topics: parsed.data.topics ?? current.topics,
-    };
-    await tx.user.update({
-      where: { id: user.id },
-      data: { previewFeaturesJson: toJson(merged) },
-    });
-    return merged;
-  });
-
-  annotate({
-    action: { name: "settings.preview-features.update" },
-    meta: { enabled: enabledFlags(next) },
-  });
-  return apiSuccess(next);
-});