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

.forgejo/workflows/ci.yml

@@ -83,8 +83,6 @@ jobs:
   backend-test:
     name: Backend Tests
     runs-on: lxc
-    # Disabled for now since tests are not integrated yet
-    if: false
     steps:
       - name: Checkout code
         uses: actions/checkout@v4