docs: reorganize into design, security, api, ops and dev; drop stale designs; refresh the diagrams [CI]

- Group docs by category with an index (docs/README.md); fix every link,
  code comment and the protocol conformance test that read docs/protocol.md.
- access.md and security.md rewritten to what is implemented: removed the
  unadopted member-proof chain, roles and recovery key that do not exist,
  the old implementation order and the budgets; gaps now carry a status.
- enrollment, deployment, api: the install flow is the static, hash-checked
  script with environment variables; no default admin.
- README and architecture: new topology and update-flow diagrams (operator
  passkey, CI signing, registry feed, auth and trust modules).

backend/tests/MoShiro.Backend.Tests/OpenApiConformanceTests.cs

@@ -4,7 +4,7 @@ using System.Text.RegularExpressions;
 namespace MoShiro.Backend.Tests;
 
 /// <summary>
-/// 对照 docs/api-guidelines.md 检查运行时生成的 OpenAPI 文档：operationId、路径风格、错误响应格式、字段命名。
+/// 对照 docs/api/api-guidelines.md 检查运行时生成的 OpenAPI 文档：operationId、路径风格、错误响应格式、字段命名。
 /// 新增端点违反规范时，这里会直接失败（等价于文档里的 <c>eng/check-openapi</c>）。
 /// </summary>
 [Collection("backend")]