fix: root redirect uses app_root_path with trailing slash

[ESnark]

🐛 Bug-fix PR

Before opening this PR please:

1. make lint - passes ruff, mypy, pylint
2. make test - all unit + integration tests green
3. make coverage - ≥ 90 %
4. make docker docker-run-ssl or make podman podman-run-ssl
5. Update relevant documentation.
6. Tested with sqlite and postgres + redis.
7. Manual regression no longer fails. Ensure the UI and /version work correctly.

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📌 Summary

Root path redirect (/ → /admin/) was ignoring APP_ROOT_PATH setting and causing unnecessary 307 redirect.

🔁 Reproduction Steps

1. Set APP_ROOT_PATH=/api
2. Request GET /api
3. Observe redirect to /admin/ instead of /api/admin/
4. Also observe 303 → 307 double redirect due to missing trailing slash

🐞 Root Cause

root_redirect function used request.scope.get("root_path", "") which returns empty string, while other parts of the codebase use settings.app_root_path. Also missing trailing slash caused extra 307 redirect.

💡 Fix Description

• Changed from request.scope.get("root_path") to settings.app_root_path for consistency (missing from fix: Export virtual server URLs now respect APP_ROOT_PATH configuration #1332 )
• Added trailing slash (/admin/) to avoid 307 redirect

🧪 Verification

Check	Command	Status
Lint suite	make lint
Unit tests	make test
Coverage ≥ 90 %	make coverage
Manual regression no longer fails	steps / screenshots

📐 MCP Compliance (if relevant)

• Matches current MCP spec
• No breaking change to MCP clients

✅ Checklist

• Code formatted (make black isort pre-commit)
• No secrets/credentials committed