Verify signature ownership in esign accept/record-event flows

Any self-provisioned trust center visitor could accept another
visitor's NDA signature or inject audit-trail events into it by
supplying its GID, since AcceptSignature and RecordEvent trusted the
client-supplied signature ID without checking it belonged to the
caller (GHSA-22xj-f767-ppw6). SignerEmail/ActorEmail are always
derived from the verified session identity, never client input, so
comparing them against the signature's stored SignerEmail in
pkg/esign/service.go closes the hole at its root without touching the
resolver-level authorization already in place elsewhere.

Adds an e2e regression test that self-provisions two trust center
visitors through the real magic-link flow and confirms one cannot
touch the other's signature.

Signed-off-by: Bryan Frimin <bryan@probo.com>

e2e/trust/main_test.go

@@ -0,0 +1,31 @@
+// Copyright (c) 2026 Probo Inc <hello@probo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package trust_test
+
+import (
+	"os"
+	"testing"
+
+	"go.probo.inc/probo/e2e/internal/testutil"
+)
+
+func TestMain(m *testing.M) {
+	testutil.Setup()
+
+	code := m.Run()
+
+	testutil.Teardown()
+	os.Exit(code)
+}