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

routes/api/v1/data/index.dart

@@ -3,7 +3,6 @@ import 'dart:io';
 
 import 'package:dart_frog/dart_frog.dart';
 import 'package:ht_api/src/registry/model_registry.dart';
-import 'package:ht_api/src/services/user_preference_limit_service.dart';
 import 'package:ht_data_repository/ht_data_repository.dart';
 import 'package:ht_shared/ht_shared.dart';