refactor(api): adapt GET /data to use native document query

- Rewrites the `_handleGet` method in the collection route handler
  to accept a single, JSON-encoded `filter` parameter.
- Removes the complex logic for translating individual URL parameters
  into a query map.
- Eliminates the `_camelToSnake` helper, as the API now passes
  native model field names to the data layer.
- This change simplifies the API handler and aligns it with a
  document-native query model, making it more flexible and robust.

Author: fulleni