feat(infra/domains): 域名池 + CDN 前置 + 签名端点分发 (tsk_NU9JuUweHWMt)

- domains.md: 四组域名隔离登记 + 冷备池 ≥5 + 启用流程（不含身份信息）
- cdn/terraform: Cloudflare 配置即代码（WAF/bot/速率限制/代理DNS/回源鉴权注入）+ 30min 重放 Runbook
- server/internal/originauth: 回源鉴权中间件，非 CDN 网段或鉴权头不符一律 403，支持双值轮换
- tools/endpoint-signer: 离线 Ed25519 签名 CLI（端点 + 公告文档，单调版本防回滚，key_id 双公钥轮换）
- tools/publish-mirrors: ≥3 镜像发布 + hash 一致性校验 + 故障转移取回
- CLIENT-CONTRACT.md: schema/验签/防回滚/合并/兜底链/channel 客户端契约
- 出站独立出口要求写入部署文档；私钥/token/身份信息一律不入库

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

infra/domains/cdn/terraform/dns.tf

@@ -0,0 +1,15 @@
+# Proxied DNS records. proxied = true keeps the origin IP hidden behind the CDN
+# (doc/05 §2 源站 IP 隐藏). The origin must NEVER have had an unproxied A record
+# (historic DNS is the most common leak path).
+resource "cloudflare_record" "api" {
+  for_each = toset(var.api_hostnames)
+
+  zone_id = var.zone_id
+  name    = each.value
+  type    = "A"
+  content = var.origin_ip
+  proxied = true
+  ttl     = 1 # 1 = automatic (required when proxied)
+
+  comment = "pangolin api/distribution endpoint (managed by terraform)"
+}