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/export/__tests__/account-settings-backup.test.ts

@@ -111,6 +111,21 @@ describe("admitAccountSettings", () => {
     expect(admitAccountSettings({}, ctx)).toEqual({ data: {}, refused: [] });
   });
 
+  it("accepts and ignores the retired preview-feature opt-ins of an older file", async () => {
+    // Written by v1.44 and v1.45, whose column v1.46 dropped. The file must
+    // still restore, so the field is neither refused nor written.
+    const { accountSettingsBackupSchema } =
+      await import("@/lib/export/account-settings-backup-schema");
+    const entry = accountSettingsBackupSchema.parse({
+      timezone: "Europe/Berlin",
+      previewFeaturesJson: { topics: true },
+    });
+    expect(admitAccountSettings(entry, ctx)).toEqual({
+      data: { timezone: "Europe/Berlin" },
+      refused: [],
+    });
+  });
+
   it("refuses a language, unit or time zone this release does not know", () => {
     const { data, refused } = admitAccountSettings(
       {

src/lib/export/account-settings-backup-schema.ts

@@ -66,6 +66,9 @@ export const accountSettingsBackupSchema = z
     moodTagLayoutJson: z.unknown().optional(),
     documentsLayoutJson: z.unknown().optional(),
     moduleListLayoutJson: z.unknown().optional(),
+    // Retired in v1.46.0 with its column (the preview-feature opt-ins). A file
+    // written by v1.44 or v1.45 still carries it; it is accepted and ignored,
+    // because no codec reads it, so such a file restores everything else.
     previewFeaturesJson: z.unknown().optional(),
     activityGoalsJson: z.unknown().optional(),
     reportSelectionJson: z.unknown().optional(),

src/lib/export/account-settings-backup.ts

@@ -539,7 +539,6 @@ const ACCOUNT_SETTING_CODECS = {
   moodTagLayoutJson: json("moodTagLayoutJson"),
   documentsLayoutJson: json("documentsLayoutJson"),
   moduleListLayoutJson: json("moduleListLayoutJson"),
-  previewFeaturesJson: json("previewFeaturesJson"),
   activityGoalsJson: activityGoals,
   reportSelectionJson: json("reportSelectionJson"),
   globalExcludedInjectionSites: excludedSites,