WIP: align both plugins with new backstage modern backend

[DevelopmentCats]

Adding support for Backstage's new backend system (v1.20+) while keeping the old backend working too.

• New module.ts file for new backend plugin system
• Moved OAuth callback from /api/auth/coder to /api/coder (the /api/auth/* path expects a full auth provider implementation, which Coder doesn't have yet)
• Updated README with instructions for both backend systems
• Frontend updated to match new callback path

[Copilot]

Pull Request Overview

This PR refactors the OAuth callback endpoint path from /api/auth/coder/oauth/callback to /api/coder/oauth/callback and adds support for Backstage's new backend system (v1.20+) while maintaining backward compatibility with the legacy backend system.

Key changes:

• Updates OAuth redirect URIs in both frontend and backend components to use the new /api/coder/oauth/callback path
• Introduces a new backend plugin module (module.ts) for the new Backstage backend system with proper authentication policies
• Adds comprehensive documentation for both new and legacy backend system setup

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
plugins/backstage-plugin-coder/src/components/CoderAuthForm/CoderAuthInputForm.tsx	Updates frontend OAuth redirect URI to match new backend path
plugins/backstage-plugin-coder-backend/src/service/router.ts	Updates backend OAuth redirect URI construction to match new path
plugins/backstage-plugin-coder-backend/src/module.ts	New module implementing backend plugin for new Backstage system with auth policies
plugins/backstage-plugin-coder-backend/src/index.ts	Exports new module alongside legacy createRouter for backward compatibility
plugins/backstage-plugin-coder-backend/package.json	Adds @backstage/backend-plugin-api dependency for new backend system support
plugins/backstage-plugin-coder-backend/README.md	Documents setup instructions for both new and legacy backend systems

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