refactor: standardize root_path access to settings.app_root_path

[Yosiefeyob]

🔗 Related Issue

Closes #1588

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📝 Summary

What does this PR do and why?
Replaces all request.scope.get("root_path", "") calls with settings.app_root_path across files.

Redirect URLs to /admin were also updated to use a consistent trailing slash (/admin/) to avoid unnecessary 307 redirects from FastAPI's path normalization.

Tests were updated to patch settings.app_root_path directly instead of injecting root_path via the request scope, which aligns with the new standardized pattern and makes tests easier to maintain.

The existing _resolve_root_path helper in admin.py was preserved as-is since it intentionally reads from the request scope first (for proxy/mounted deployments) and falls back to settings.app_root_path — this links to PR #1547.

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🏷️ Type of Change

• Bug fix
• Feature / Enhancement
• Documentation
• Refactor
• Chore (deps, CI, tooling)
• Other (describe below)

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🧪 Verification

Check	Command	Status
Lint suite	make lint	✅
Unit tests	make test	✅
Coverage ≥ 80%	make coverage	✅

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✅ Checklist

• Code formatted (make black isort pre-commit)
• Tests added/updated for changes
• Documentation updated (if applicable)
• No secrets or credentials committed

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[crivetimihai]

Thanks @Yosiefeyob. Clean refactor — using request.scope.get("root_path", "") is more resilient and aligns with ASGI conventions. Please add the DCO Signed-off-by line to your commit.