docs: clarify PYTHON_BIN validation rules in env example.

web/server/.env.example

@@ -7,5 +7,7 @@ GROQ_API_KEY=your_groq_api_key_here
 # Server port (optional, defaults to 5000)
 PORT=5000
 
-# Python bin (for venv); optional
-PYTHON_BIN=/path/to/bin/python
+# Python interpreter for the retrieval daemon (optional, defaults to "python").
+# Must be "python", "python3", or an absolute path to a Python 3 executable.
+# Do not set this to an arbitrary binary -- the value is validated on startup.
+# PYTHON_BIN=/path/to/venv/bin/python3