Install into a virtual environment

Raspberry Pi OS refuses pip install into the system Python, so the venv
step is the difference between the README working and not. The systemd
unit points at that interpreter for the same reason.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -25,12 +25,24 @@ Any other mix works — it is all in `config.py`.
 ## Install
 
 ```bash
-pip3 install -r requirements.txt
+cd /home/pi/heos
+python3 -m venv .venv
+source .venv/bin/activate
+pip install -r requirements.txt
 python3 app.py
 ```
 
 Then open `http://<pi-ip>:5005/`.
 
+The virtual environment is not optional on a current Raspberry Pi OS:
+`pip install` straight into the system Python is refused there with
+`externally-managed-environment`. It also keeps Flask out of the way of
+anything else running on the Pi.
+
+Every later `python3 ...` command here assumes the environment is active
+(`source .venv/bin/activate`); `deactivate` when you are done. The service
+below calls the environment's Python directly, so it does not care.
+
 ## Configure
 
 Open `http://<pi-ip>:5005/api/targets` and copy the exact `name` HEOS reports
@@ -70,7 +82,7 @@ Description=HEOS panel
 After=network-online.target
 
 [Service]
-ExecStart=/usr/bin/python3 /home/pi/heos/app.py
+ExecStart=/home/pi/heos/.venv/bin/python /home/pi/heos/app.py
 WorkingDirectory=/home/pi/heos
 Restart=always
 User=pi