docs: clarify Python deps use apt packages not pip/venv

Dev server uses apt-managed python3-pandas and python3-openpyxl.
Production fix is the same. Updates README install step and rewrites
python-venv-setup.md to reflect the real setup with venv as fallback.

README.md

@@ -68,7 +68,7 @@ The application provides:
 
 - Node.js 18 or later
 - npm
-- Python 3 with a venv containing `pandas` and `openpyxl` (required for compliance xlsx parsing)
+- Python 3 with `python3-pandas` and `python3-openpyxl` apt packages (required for compliance xlsx parsing)
 
 ---
 
@@ -97,28 +97,13 @@ npm install
 
 ### 4. Install Python dependencies
 
-Modern Debian/Ubuntu systems enforce PEP 668 and block system-wide pip installs. Create a virtual environment instead:
+Install via apt — this is the correct approach on Ubuntu/Debian and mirrors the dev server setup:
 
 ```bash
-# Install venv support if needed
-apt install -y python3-venv python3-full
-
-# Create the venv (once per server, from the app root)
-python3 -m venv /home/cve-dashboard/venv
-
-# Install packages into the venv
-/home/cve-dashboard/venv/bin/pip install -r backend/scripts/requirements.txt
+apt install -y python3-pandas python3-openpyxl
 ```
 
-Required packages: `pandas>=2.0.0`, `openpyxl>=3.0.0`
-
-Then set the `PYTHON_BIN` environment variable so the backend uses the venv Python:
-
-```bash
-export PYTHON_BIN=/home/cve-dashboard/venv/bin/python3
-```
-
-Add this to the server's startup environment (e.g., your systemd unit or `.env` file) so it persists across restarts. If `PYTHON_BIN` is not set, the backend falls back to the system `python3`.
+> If apt packages are unavailable or you need a specific version, see `docs/python-venv-setup.md` for the venv fallback approach.
 
 > The bulk notes import script (`import_notes_from_csv.py`) uses only Python stdlib and does **not** require these packages.