FEATURE: Add category ignore list for GitHub linkbacks

[ducks]

Adds github_linkback_ignored_categories site setting to prevent linkback comments from being posted to GitHub when posts are created in specific categories.

This is useful for filtering out noise from automated posts (e.g. AI summaries, automated reports) that reference GitHub URLs but don't need linkback comments.

The setting accepts a list of category IDs. Posts in these categories will skip the linkback creation entirely, following the same pattern as the existing github_linkback_projects setting

Screenshot of new setting:

Ref: /t/141863