Move dev servers to ports 8002 and 5172

Backend now runs on 8002 and the frontend dev server on 5172, with the
proxy, CORS origins and the run_*.bat launchers updated to match. The
launchers also activate the venv so they work from a clean shell.

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

frontend/.env.example

@@ -1 +1,2 @@
-REACT_APP_API_URL=http://localhost:8000
+PORT=5172
+REACT_APP_API_URL=http://localhost:8002