fix(ci): scope rustls security waiver

Temporarily waive RUSTSEC-2026-0285 until 2026-09-21 UTC to preserve the pinned Codex TLS fingerprint. Keep all other audit failures blocking.

Validated cargo audit and waiver expiry behavior.

Co-authored-by: Codex <noreply@openai.com>

.github/workflows/security-scan.yml

@@ -40,7 +40,16 @@ jobs:
 
       - name: Run cargo audit
         working-directory: backend
-        run: cargo audit
+        shell: bash
+        run: |
+          # 临时接受此中危风险以保持 Codex 0.154.0 的 TLS 指纹；直接升级会新增 ML-DSA 签名算法。
+          # 仅豁免本条公告，2026-09-21 UTC 起恢复阻断；兼容升级验证完成后应提前删除豁免。
+          # https://github.com/rustls/rustls/security/advisories/GHSA-2mjx-qc3c-rqvc
+          audit_advisory_args=()
+          if [[ "$(date -u +%F)" < "2026-09-21" ]]; then
+            audit_advisory_args+=(--ignore RUSTSEC-2026-0285)
+          fi
+          cargo audit "${audit_advisory_args[@]}"
 
   frontend-security:
     runs-on: ubuntu-latest