Add SecureBackupSetupEntryPoint extension seam

Introduces a FeatureEntryPoint for the recovery-key setup screen so enterprise builds can vend a richer setup node. The FOSS default builds the standard auto-generated-key node; SecureBackupFlowNode now resolves the Setup and Change targets through this seam instead of creating SecureBackupSetupNode directly.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

features/securebackup/api/src/main/kotlin/io/element/android/features/securebackup/api/SecureBackupSetupEntryPoint.kt

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) 2026 Element Creations Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+package io.element.android.features.securebackup.api
+
+import com.bumble.appyx.core.modality.BuildContext
+import com.bumble.appyx.core.node.Node
+import io.element.android.libraries.architecture.FeatureEntryPoint
+import io.element.android.libraries.architecture.NodeInputs
+
+/**
+ * Entry point for the recovery-key setup screen.
+ *
+ * The default (FOSS) implementation builds the standard auto-generated-key setup node. Enterprise
+ * builds replace this binding to vend a richer setup node (e.g. a homeserver-advertised custom
+ * recovery passphrase) while falling back to the standard node when no custom spec applies.
+ */
+interface SecureBackupSetupEntryPoint : FeatureEntryPoint {
+    data class Inputs(val isChangeRecoveryKeyUserStory: Boolean) : NodeInputs
+
+    fun createNode(
+        parentNode: Node,
+        buildContext: BuildContext,
+        inputs: Inputs,
+    ): Node
+}