Address PR review comments

- Make ExtensionPointFactoryMiddleware an opaque type via new
  @internal/backend package using OpaqueType from @internal/opaque
- Use options object for createExtensionPointFactoryMiddleware
- Make middleware function async (returns Promise<T>)
- Remove extensionPointFactoryMiddleware from createBackend, keep only
  on createSpecializedBackend
- Export defaultServiceFactories from @backstage/backend-defaults

Signed-off-by: Jack Palmer <jackpalmer@spotify.com>

packages/backend-internal/.eslintrc.js

@@ -0,0 +1,5 @@
+module.exports = require('@backstage/cli/config/eslint-factory')(__dirname, {
+  rules: {
+    '@backstage/no-top-level-material-ui-4-imports': 'error',
+  },
+});

packages/backend-internal/README.md

@@ -0,0 +1,3 @@
+# @internal/backend
+
+This is an internal package used by the other backend packages. It does not get published to NPM, but instead inlined into consuming packages due to the `backstage.inline` flag in `package.json`.

packages/backend-internal/catalog-info.yaml

@@ -0,0 +1,9 @@
+apiVersion: backstage.io/v1alpha1
+kind: Component
+metadata:
+  name: internal-backend
+  title: '@internal/backend'
+spec:
+  lifecycle: experimental
+  type: backstage-node-library
+  owner: framework-maintainers

packages/backend-internal/package.json

@@ -0,0 +1,28 @@
+{
+  "name": "@internal/backend",
+  "version": "0.0.1",
+  "backstage": {
+    "role": "node-library",
+    "inline": true
+  },
+  "private": true,
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/backstage/backstage",
+    "directory": "packages/backend-internal"
+  },
+  "license": "Apache-2.0",
+  "sideEffects": false,
+  "main": "src/index.ts",
+  "types": "src/index.ts",
+  "files": [
+    "dist"
+  ],
+  "scripts": {
+    "lint": "backstage-cli package lint",
+    "test": "backstage-cli package test"
+  },
+  "devDependencies": {
+    "@backstage/cli": "workspace:^"
+  }
+}

packages/backend-internal/src/index.ts

@@ -0,0 +1,17 @@
+/*
+ * Copyright 2026 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+export { OpaqueExtensionPointFactoryMiddleware } from './wiring';

packages/backend-internal/src/wiring/OpaqueExtensionPointFactoryMiddleware.ts

@@ -0,0 +1,29 @@
+/*
+ * Copyright 2026 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { OpaqueType } from '@internal/opaque';
+
+export const OpaqueExtensionPointFactoryMiddleware = OpaqueType.create<{
+  public: { $$type: '@backstage/ExtensionPointFactoryMiddleware' };
+  versions: {
+    readonly version: 'v1';
+    readonly extensionPointId: string;
+    readonly middleware: (original: unknown) => Promise<unknown>;
+  };
+}>({
+  type: '@backstage/ExtensionPointFactoryMiddleware',
+  versions: ['v1'],
+});

packages/backend-internal/src/wiring/index.ts

@@ -0,0 +1,17 @@
+/*
+ * Copyright 2026 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+export { OpaqueExtensionPointFactoryMiddleware } from './OpaqueExtensionPointFactoryMiddleware';