fix(compliance): use PYTHON_BIN env var for venv support

Modern Debian/Ubuntu enforces PEP 668 which blocks system-wide pip
installs. The backend now reads PYTHON_BIN from the environment
(defaulting to 'python3') so each server can point to a venv.
Updates README with venv setup instructions.

backend/routes/compliance.js

@@ -17,6 +17,7 @@ const fs       = require('fs');
 const { spawn } = require('child_process');
 
 const PARSER_SCRIPT = path.join(__dirname, '../scripts/parse_compliance_xlsx.py');
+const PYTHON_BIN    = process.env.PYTHON_BIN || 'python3';
 const TEMP_DIR      = path.join(process.cwd(), 'uploads', 'temp');
 const ALLOWED_TEAMS = new Set(['STEAM', 'ACCESS-ENG', 'ACCESS-OPS', 'INTELDEV']);
 
@@ -47,7 +48,7 @@ function dbAll(db, sql, params = []) {
 // ---------------------------------------------------------------------------
 function parseXlsx(filePath) {
     return new Promise((resolve, reject) => {
-        const py = spawn('python3', [PARSER_SCRIPT, filePath]);
+        const py = spawn(PYTHON_BIN, [PARSER_SCRIPT, filePath]);
         let out = '';
         let err = '';
         py.stdout.on('data', d => { out += d; });