feat(ECR-042): OpsCMS FeedSlot 薄写面

Admin POST/PUT 复用 admin.cms.write；C 端 GET /home/feed-slots；首页无 active 槽隐藏推荐区、失败回退展示；无新 migration。

Co-authored-by: Cursor <cursoragent@cursor.com>

docs/CONTRACT_DIFF/ECR-042.yaml

@@ -0,0 +1,19 @@
+ecr: ECR-042
+capability: OpsCMS
+change:
+  type: additive
+breaking_change: false
+migration_required: false
+apis:
+  - method: POST
+    path: /api/v1/admin/cms/feed-slots
+    change: added
+  - method: PUT
+    path: /api/v1/admin/cms/feed-slots/{id}
+    change: added
+  - method: GET
+    path: /api/v1/home/feed-slots
+    change: added
+perms:
+  - code: admin.cms.write
+    change: reused