[P1] Placeholder URL template with literal <HOST>/<PORT> in chatqna response

Problem

When constructing source document URLs for the response, the ChatQnA service uses a placeholder template https://<HOST>/<PORT>/api/files/{file_id}/viewbrowser with literal <HOST> and <PORT> strings. This produces broken URLs in the API response.

Files

• chatqna/genieai_chatqna.py:1505 — file_read_url = f"https://<HOST>/<PORT>/api/files/{file_id}/viewbrowser"

Impact

Users clicking on source document links receive broken URLs. This also affects citation accuracy and the frontend's ability to display source documents.

Acceptance Criteria

• Replace the placeholder with an environment variable (e.g., DOC_REPO_PUBLIC_URL)
• Set a sensible default or fail fast if the URL is not configured
• Verify that the generated URLs are valid and reachable
• Add validation that the URL template does not contain literal placeholders