add pod annotations to digger-backend helm chart

[windrammer]

This PR will add the possibility to set pod annotations in the backend deployment.

✅ AI Disclosure Checklist

• I understand that all AI assistance must be disclosed.
• I did not use AI tools in this contribution.
• I used AI tools and have disclosed details below.

Details:
Github copilot gpt-4.1 used for autocompletion
I know that doesn't require disclosure but with such a small PR i figure I might as well.

---

💡 Notes

• Trivial auto-completions (single words, short phrases) don’t need disclosure.
• Contributors must understand and take responsibility for any AI-assisted code.
• Failure to disclose is considered disrespectful to maintainers and may delay review.