test(auth): add multi-user regression coverage

- Enable backend tests in CI (remove if: false)
- Fix test_products_helpers.py to pass current_user parameter
- Fix test_routines_helpers.py to include short_id in products
- Fix llm_context.py to use product_effect_profile correctly
- All 221 tests passing

.sisyphus/evidence/task-T11-ci-enabled.txt

@@ -0,0 +1,6 @@
+  backend-test:
+    name: Backend Tests
+    runs-on: lxc
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4