fix(server): ghost 401 must abort the chain — c.JSON's nil error let the request continue

The guard returned c.JSON(401, ...), whose nil error told echo the
SuccessHandler succeeded — the pipeline then continued into the route
handler, which type-asserted the never-set "user" context value and
panic-recovered as a 500 appended after the 401 body. The status code
happened to survive (first write wins), which is why the pin test
passed, but every ghost request also executed the target handler with
no user — the exact unauthenticated-execution class the guard exists to
prevent.

Return echo.NewHTTPError(401, ...) instead: non-nil aborts the chain
and renders {"message":"user_not_found: ..."}; the pin test now asserts
that shape. Full suite: 219 groups, zero failures.

cmd/server/tests/progress_service_test.go

@@ -717,7 +717,7 @@ func TestDeletedUserTokenRejected(t *testing.T) {
 
 	var errBody map[string]string
 	require.NoError(t, json.NewDecoder(resp.Body).Decode(&errBody))
-	assert.Equal(t, "user_not_found", errBody["error"])
+	assert.Contains(t, errBody["message"], "user_not_found")
 }
 
 // TestProgressWeb_LocatorCoherence pins the locator coherence rule: a