feat(dashboard): refactor load path and usage refresh

[Soju06]

Summary

• move usage refresh to background scheduler and remove request-path refresh
• add dashboard overview endpoint and supporting dashboard module
• slim account/usage payloads to UI-needed fields only
• refactor frontend to use overview endpoint and lazy-load options/settings

Testing

• .venv/bin/python -m pytest

[chatgpt-codex-connector]

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[Soju06]

/codex review

[Soju06]

@codex review

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 504fdb8d45

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".