Closes #24
This merge request adds a public provider catalogue endpoint for GeMeDa integrations.
Changes:
- add the maintained provider catalogue in backend/gemeda/config/providers.json;
- validate and load the catalogue in the backend;
- expose GET /api/v1/providers;
- return stable provider IDs, display names and existing metasearch IDs;
- document the endpoint, response format and caching guidance.
The endpoint exposes provider metadata only and does not require authentication.