Add OpenAPI docs with examples and status codes

[GusAlberto]

Adds Scramble-based OpenAPI/Swagger documentation for the product API.

Included:

• Generated docs UI at /docs/api and JSON at /docs/api.json
• Query parameter examples for listing and filtering products
• Request payload examples for create/update endpoints
• Explicit response status documentation for success responses
• README instructions for accessing the generated docs

Validation:

• php artisan test --testsuite=Feature