refactor: remove the runtime binary binding; every release registration path verifies the CI signature [CI]

The hash a node reports about itself cannot be trusted (a swapped binary can
copy the old hash), so the runtime binding only caught files that were replaced
by something that did not lie. Provenance stays where it can be verified: the
signed release manifest (digest) and the signed SHA256SUMS.

- Remove BinaryBindingService, the node_bindings table (schema v15 drops it),
  BinaryPolicy, the handshake hash/manifest fields and the admin binary column.
- Drop the per-platform "binaries" list from the release manifest, the hash
  scripts and the updater's installed.manifest copy.
- ReleaseService.PublishAsync now verifies the release signature, so the admin
  POST /releases route is no longer an unsigned path; tests sign with a test key.
- Docs: section 16 explains why the runtime check is gone.

docs/api.md

@@ -150,7 +150,6 @@
 | POST | `/api/release/offer` | CI 登记发布清单的入口，**不需要令牌**：`{manifest, signature}` 的签名必须能用 Backend 内嵌的发布公钥验过（ECDSA P-256 / SHA-256，DER），否则 403；`seq` 必须比已登记的新，否则 409。登记不依赖面板里的任何凭证，清库、重装、还没有 passkey 时照常工作 |
 | GET | `/api/admin/links` | 每一对（Probe，Agent）的线路开关：`linked`（最终结果）、`override`（手动设置，`null` 跟随自动连接）、`connected`，以及 `auto_link` |
 | PUT | `/api/admin/links/{probe_id}/{agent_id}` | `{enabled: true｜false｜null}`：手动强制连或不连，`null` 取消手动设置；改完立即重新下发 |
-| DELETE | `/api/admin/nodes/{id}/binary` | 接受节点当前的二进制：清除它的二进制绑定，下一次握手重新绑定（需要 `fleet.write`；没有绑定也返回 204）。`GET /api/admin/build-info` 的每个节点带 `binary`（`sha256`、`signed`、`seq`、`bound_at`、`violation`） |
 | POST | `/api/admin/nodes/{id}/update` | 请求一个在线的 Agent 或 Probe 更新到它收到的清单（节点不在线返回 409） |
 | GET/POST/PATCH | `/api/admin/users` | 首次启动时通过命令行或环境变量创建初始管理员 |