Defines REPO_ROOT = Path(__file__).parents[3] once in core.py.
Replaces Path(__file__).parent.parent...parent chains and Path("data/...")
CWD-relative references in admin/routes.py, content/__init__.py,
content/routes.py, and worker.py (4x local repo_root variables).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>