feat: trusted release keys setting; drop the duplicate release registration route [CI]

The panel now lets the deployer decide which release keys it trusts. The
official CI key is built in and always trusted; an own P-256 public key can be
added (off by default, the private key is rejected) so manifests signed with it
are accepted too. Removing it drops the manifests only it signed. Changes are
audited and need update.confirm.

Admin UI: System > Trust > "Trusted release keys" with a collapsible guide on
generating a key pair and signing a manifest with openssl.

Registration now has one path, POST /api/release/offer (no token, signature
verified). The admin POST /releases route, the release.publish capability and
the ci-release preset are removed.

backend/src/MoShiro.Backend.Api/Admin/BuildEndpoints.cs

@@ -36,15 +36,6 @@ public static class BuildEndpoints
             .Requires(Capability.UpdateRead);
     }
 
-    /// <summary>登记发布清单：挂在 <see cref="ReleaseGuard"/> 下，除了管理员，CI 用的 <c>release</c> 令牌也能调用（它只能做这一件事）。</summary>
-    public static void MapPublish(RouteGroupBuilder group)
-    {
-        group.MapPost("/releases", PublishReleaseAsync)
-            .Api("releases_publish", "Register a CI-signed release manifest (the signature is checked against the release key); online nodes get it right away. Admin or release tokens", Tag)
-            .Returns<ReleaseDto>(StatusCodes.Status201Created).ProducesProblems(400, 401, 403, 409)
-            .Requires(Capability.ReleasePublish);
-    }
-
     /// <summary>
     /// CI 登记清单的入口，<b>不需要令牌</b>：请求里的清单必须带有效的 CI 发布签名（用 Backend 内嵌的发布公钥验）才收下。
     /// 这样登记不依赖面板里的任何凭证（清库、重装、还没有 passkey 的时候照常工作），也没有一把能被偷的长期令牌；
@@ -57,12 +48,12 @@ public static class BuildEndpoints
             .Returns<ReleaseDto>(StatusCodes.Status201Created).ProducesProblems(400, 403, 409);
     }
 
-    private static async Task<IResult> OfferReleaseAsync(HttpContext http, ReleaseService releases, IReleaseKey releaseKey, PublishReleaseRequest request)
+    private static async Task<IResult> OfferReleaseAsync(HttpContext http, ReleaseService releases, ReleaseKeyService keys, PublishReleaseRequest request)
     {
         string manifest = request.Manifest ?? "";
         string signature = request.Signature ?? "";
         if (manifest.Length is 0 or > ReleaseService.MaxManifestBytes || signature.Length is 0 or > ReleaseService.MaxSignatureBytes
-            || !ReleaseSignature.Verify(releaseKey.Pem, manifest, signature))
+            || !await keys.VerifyAsync(manifest, signature, http.RequestAborted))
         {
             return Problems.Create(http, MoShiroErrors.Forbidden, "the manifest does not carry a valid release signature");
         }