feat: install nodes and the backend from a static compose file; remove dead code [CI]

Agent, Probe and Backend now start from one static compose.yaml plus a local
.env: the panel (or the release assets, for the very first Backend) provides
the file, the panel's command writes the public values into .env, checks the
file's SHA-256 and runs compose up. Updating is compose pull && compose up -d.
The image is chosen by the CI channel (:latest, or MOSHIRO_VERSION in .env),
not by the panel; the previous one-line script stays as a fallback.

- packaging/compose/{agent,probe,backend}/compose.yaml are the single source,
  embedded in the Backend and served at /install/compose/{file}; the root
  compose.yaml moved there. Releases also carry them and list them in the
  signed SHA256SUMS.
- The container entrypoint defaults the node name to the host name.
- Join token creation returns a compose block (.env, url, hash, per-platform
  commands); the admin dialog leads with it.
- Remove code with no references (HotLayer members, LiveHub.SubscriberCount,
  QueryService.ParseParams, ReleaseService.Forget, a few constants) and reduce
  the install-code lookup to the role it is for.

packaging/compose/backend/compose.yaml

@@ -0,0 +1,44 @@
+# MoShiro Backend（面板）。Docker 和 rootless/rootful Podman 共用。
+#
+# 用法：
+#   mkdir moshiro-backend && cd moshiro-backend
+#   下载本文件为 compose.yaml；写一个 .env（可选）：
+#     MOSHIRO_PUBLIC_URL=https://panel.example.com     # 面板对外的地址；passkey 绑定在这个域名上，改了会让已注册的 passkey 失效
+#     MOSHIRO_CLAIM_CODE=                              # 认领码，不写就随机生成并打印在日志里（podman logs moshiro-backend）
+#   podman compose up -d            # Docker 用 docker compose
+#   第一次打开面板按页面引导认领：没有默认管理员。面板监听 127.0.0.1:8080，对外请放在反向代理（HTTPS）后面。
+# 更新：先备份数据卷，再 podman compose pull && podman compose up -d。新版会自动迁移数据库，
+#   但旧版不认比自己新的数据库：想回滚要先恢复备份，只改 MOSHIRO_VERSION 不够。
+# 说明见 docs/ops/deployment.md。
+services:
+  backend:
+    image: registry.oeo.one/${MOSHIRO_IMAGE_NAMESPACE:-moshiro}/moshiro-backend:${MOSHIRO_VERSION:-latest}
+    container_name: moshiro-backend
+    restart: unless-stopped
+    init: true
+    ports:
+      - "127.0.0.1:8080:8080"
+    environment:
+      ASPNETCORE_HTTP_PORTS: "8080"
+      MoShiro__DataDirectory: /data
+      MoShiro__UpdateChannel: ${MOSHIRO_UPDATE_CHANNEL:-auto}
+      MoShiro__ImageNamespace: ${MOSHIRO_IMAGE_NAMESPACE:-moshiro}
+      MoShiro__PublicUrl: "${MOSHIRO_PUBLIC_URL:-}"
+      MoShiro__ClaimCode: "${MOSHIRO_CLAIM_CODE:-}"
+    volumes:
+      - moshiro-backend-data:/data
+      - moshiro-backend-status:/data/update-status:ro
+    read_only: true
+    tmpfs:
+      - /tmp:rw,noexec,nosuid,size=64m,mode=1777
+    security_opt:
+      - no-new-privileges:true
+    cap_drop:
+      - ALL
+    stop_grace_period: 30s
+
+volumes:
+  moshiro-backend-data:
+    name: moshiro-backend-data
+  moshiro-backend-status:
+    name: moshiro-backend-status