feat(ECR-041): OpsCMS Banner 薄写面（Write-Wave 首刀）

Admin POST/PUT + admin.cms.write/审计；C 端 GET /home/banners；首页投影回退静态 homeFeeds；migration 000051；不碰 FeedSlot/支付/UGC。

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

docs/BACKEND_DESIGN/BD-2026-041-banner-write.md

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+# Backend Design: ECR-041 Banner Write
+
+| Field | Value |
+|-------|-------|
+| ID | BD-2026-041 |
+| Status | **Approved** |
+| Coding | **Authorized · Implemented · Closed with ECR-041** |
+| Level | L2 |
+| Migration | `000051_ops_banner_write` |
+
+## Backend Change Boundary
+
+```text
+Domain: Banner (write + C-end read)
+App: AdminHandler / HomeHandler → admin.Service / home → AdminRepo
+API:
+  POST /api/v1/admin/cms/banners
+  PUT  /api/v1/admin/cms/banners/{id}
+  GET  /api/v1/home/banners?placement=home
+Permission: admin.cms.write (additive); keep admin.cms.read
+Audit: admin_audit_logs on POST/PUT
+Table: reuse ops_banners (ECR-024)
+```
+
+## Out of boundary
+
+FeedSlot 写 · ScheduledPublication 写 · UGC · 真支付 · soft-delete · Crisis/Handoff/ContentSafety 写 · 新 RBAC 子系统
+
+## Rollback
+
+1. migration down（撤权限种子）  
+2. 移除写路由与 C 端读取路由  
+3. H5 回退静态 `homeFeeds`  
+
+## Refs
+
+Spec `ops-banner-write.md` · ECR-041 · WRITE_WAVE_AUTHORIZATION · predecessor BD-2026-024