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.

docs/api/openapi.yaml

@@ -15428,45 +15428,6 @@ paths:
         "403": *a9
         "422": *a4
         "429": *a5
-  /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:
-                $ref: "#/components/schemas/PreviewFeaturesEnvelope"
-        "401": *a1
-        "403": *a9
-        "422": *a4
-        "429": *a5
-    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:
-              $ref: "#/components/schemas/PreviewFeaturesPatchBody"
-      responses:
-        "200":
-          description: The opt-ins as stored.
-          content:
-            application/json:
-              schema:
-                $ref: "#/components/schemas/PreviewFeaturesSavedEnvelope"
-        "401": *a1
-        "403": *a9
-        "422": *a4
-        "429": *a5
   /api/settings/privacy-summary:
     get:
       tags:
@@ -27212,6 +27173,61 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
+  /api/settings/preview-features:
+    get:
+      security: []
+      deprecated: true
+      tags:
+        - Retired
+      summary: GET /api/settings/preview-features — removed in v1.46.0
+      description: >-
+        Removed in v1.46.0. Every request to this path answers 410 Gone, whatever the method and whatever credential it
+        carries. There is no replacement; the capability is gone rather than moved.
+
+
+        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.
+
+
+        Retiring a path is deliberate, so the answer says so rather than leaving a client to read a 404 as a broken
+        deployment and retry forever. Treat 410 as terminal: drop the call, degrade the surface that depended on it, and
+        do not offer a retry.
+      responses:
+        "410":
+          description: The path was removed in v1.46.0 and will not answer again. `meta.errorCode` = `route.retired`,
+            `meta.removedIn` carries the version, and `meta.replacedBy` carries the replacement path or null.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/ErrorEnvelope"
+    patch:
+      security: []
+      deprecated: true
+      tags:
+        - Retired
+      summary: PATCH /api/settings/preview-features — removed in v1.46.0
+      description: >-
+        Removed in v1.46.0. Every request to this path answers 410 Gone, whatever the method and whatever credential it
+        carries. There is no replacement; the capability is gone rather than moved.
+
+
+        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.
+
+
+        Retiring a path is deliberate, so the answer says so rather than leaving a client to read a 404 as a broken
+        deployment and retry forever. Treat 410 as terminal: drop the call, degrade the surface that depended on it, and
+        do not offer a retry.
+      responses:
+        "410":
+          description: The path was removed in v1.46.0 and will not answer again. `meta.errorCode` = `route.retired`,
+            `meta.removedIn` carries the version, and `meta.replacedBy` carries the replacement path or null.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/ErrorEnvelope"
 components:
   schemas:
     CreateCycleCustomSymptomRequest:
@@ -33264,12 +33280,6 @@ components:
         - version
         - list
       additionalProperties: false
-    PreviewFeaturesPatchBody:
-      type: object
-      properties:
-        topics:
-          type: boolean
-      additionalProperties: false
     ConsentPostBody:
       type: object
       properties:
@@ -73560,50 +73570,6 @@ components:
         - data
         - error
       additionalProperties: false
-    PreviewFeaturesEnvelope:
-      type: object
-      properties:
-        data:
-          $ref: "#/components/schemas/PreviewFeatures"
-        error:
-          type: "null"
-        meta:
-          type: object
-          properties:
-            requestId:
-              type: string
-          additionalProperties: false
-      required:
-        - data
-        - error
-      additionalProperties: false
-    PreviewFeatures:
-      type: object
-      properties:
-        topics:
-          type: boolean
-          description: Topic overview pages above the single-metric Insights pages. Off by default.
-      required:
-        - topics
-      additionalProperties: false
-      description: Per-user opt-in to preview features, one boolean per flag, every flag off by default.
-    PreviewFeaturesSavedEnvelope:
-      type: object
-      properties:
-        data:
-          $ref: "#/components/schemas/PreviewFeatures"
-        error:
-          type: "null"
-        meta:
-          type: object
-          properties:
-            requestId:
-              type: string
-          additionalProperties: false
-      required:
-        - data
-        - error
-      additionalProperties: false
     PrivacySummaryEnvelope:
       type: object
       properties: