fix: require root_url on email_auth_key_url; correct frontend guide for user auth endpoints

- Make root_url a required query param on GET /v3/action/user/{id}/email_auth_key_url
  (previously Optional[str]=None, which produced a malformed link in the emailed URL)
- Update GUIDE__AE_API_V3_for_Frontend.md: document root_url as required, add magic link
  URL format, note valid_email=True side effect, add 404 error, expand 403 conditions
  for authenticate, add 400 for verify_password when no password is set
- Add test_e2e_v3_user_action_routes.py and test_e2e_v3_user_auth_routes.py to tests/README.md

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/routers/api_v3_actions_user.py

@@ -271,7 +271,7 @@ async def action_new_auth_key(
 @router.get('/{user_id}/email_auth_key_url', response_model=Resp_Body_Base)
 async def action_email_auth_key_url(
     user_id: str = Path(min_length=11, max_length=22),
-    root_url: Optional[str] = Query(None, min_length=10, max_length=200),
+    root_url: str = Query(..., min_length=10, max_length=200),
     key_param_name: str = Query('auth_key', min_length=2, max_length=30),
     account: AccountContext = Depends(get_account_context),
 ):