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/tests/MoShiro.Backend.Tests/InstallTests.cs

@@ -85,7 +85,7 @@ public sealed class InstallTests
     public async Task TheScript_GetsTheSignedManifestOfItsRole_ByteForByte()
     {
         string manifest = """{"schema":1,"component":"probe","channel":"dev","image":"registry.example/moshiro-dev/moshiro-probe","digest":"sha256:0000000000000000000000000000000000000000000000000000000000000000","tag":"dev-1","version":"0.1.1","commit":"abc","built_at":"2026-10-06T00:00:00Z","seq":1}""";
-        using HttpResponseMessage published = await _backend.Admin.PostAsync("/api/admin/releases", BackendFixture.Json(new JsonObject { ["manifest"] = manifest, ["signature"] = TestReleaseKey.Sign(manifest) }));
+        using HttpResponseMessage published = await _backend.Anonymous.PostAsync("/api/release/offer", BackendFixture.Json(new JsonObject { ["manifest"] = manifest, ["signature"] = TestReleaseKey.Sign(manifest) }));
         Assert.True(published.StatusCode is HttpStatusCode.Created or HttpStatusCode.Conflict, await published.Content.ReadAsStringAsync());
 
         string code = (await CreateAsync("probe"))["plaintext"]!.GetValue<string>();