feat(ECR-020): QualityFeedback 问答质量反馈并 Closed

新增 ask_quality_feedback、运营/C端评分 API 与 admin-h5 问答反馈区；禁改消息/UGC/真支付。

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

docs/BACKEND_DESIGN/BD-2026-020-quality-feedback.md

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+# Backend Design: ECR-020 QualityFeedback
+
+| ID | BD-2026-020 |
+| Status | Approved |
+| Coding | Loop authorized |
+| Level | L2 |
+| Migration | YES 000021 |
+
+## Backend Change Boundary
+
+```text
+Domain: QualityFeedback
+App: AdminHandler + AskHandler → services → repos
+API: GET /admin/ask/feedback
+     POST /admin/ask/threads/{id}/feedback
+     POST /ask/threads/{id}/feedback
+Permission: admin.ask.read · admin.ask.feedback.write
+Migration: 000021
+UI: admin-h5 /ask feedback panel
+```
+
+## Out of boundary
+
+Message mutation · SystemPrompt · UGC plaza · Payment
+
+Rollback: down migration + remove routes/UI