ver. 0.302: REST API product variants, attributes dictionary, attribute filtering

- Add variant CRUD endpoints (variants, create_variant, update_variant, delete_variant)
- Add dictionaries/attributes endpoint with multilingual names and values
- Add attribute_* filter params for product list filtering by attribute values
- Enrich product detail attributes with translated names (attribute_names, value_names)
- Include variants array in product detail response for parent products
- Add price_brutto validation on product create
- Batch-load attribute/value translations (4 queries instead of N+1)
- Add 43 new unit tests (730 total, 2066 assertions)

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

docs/TESTING.md

@@ -23,7 +23,7 @@ composer test              # standard
 ## Aktualny stan
 
 ```text
-OK (692 tests, 1988 assertions)
+OK (730 tests, 2066 assertions)
 ```
 
 Zweryfikowano: 2026-02-21 (ver. 0.300)