refactor(build): define ankiBackendDependencies

[david-allison]

Note

Assisted-by: Claude Opus 4.8

Purpose / Description

I will add this to anki-common, and it's a lot of complexity

Fixes

• Part of Architecture Discussion: Module Extraction #20737

How Has This Been Tested?

App still runs; local_backend is still blocked on the Anki-Android-Backend java-library conversion

Checklist

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner