feat: add GET /auth/logout route that clears session cookie and redirects to /login

- Add @app.get('/auth/logout') handler in muxplex/main.py after POST /login
- Creates RedirectResponse('/login', status_code=303)
- Calls response.delete_cookie('muxplex_session') to clear the auth cookie
- Add two tests in test_api.py:
  - test_logout_redirects_to_login: verifies 303 status and /login in location header
  - test_logout_clears_session_cookie: verifies Set-Cookie has muxplex_session with max-age=0
- Route is already exempt from AuthMiddleware via _AUTH_EXEMPT_PATHS in auth.py

Co-authored-by: Amplifier <amplifier@sourcegraph.com>

muxplex/main.py

@@ -524,6 +524,14 @@ async def post_login(
     return response
 
 
+@app.get("/auth/logout")
+async def logout() -> RedirectResponse:
+    """Clear the muxplex_session cookie and redirect to /login."""
+    response = RedirectResponse("/login", status_code=303)
+    response.delete_cookie("muxplex_session")
+    return response
+
+
 @app.get("/auth/mode")
 async def auth_mode_endpoint():
     """Return the current auth mode and running username."""