Implement BadgeDefinitionRepository with criteria support

Define `BadgeCriteriaType` as a Dart enum with `assignmentCount` and `hourCount` values, with a static `fromString` factory that maps snake_case JSON values ('assignment_count') to enum values. Use `freezed` or a simple immutable class for `BadgeCriteria`. The `criteria` field on `BadgeDefinition` must never be nullable — default to `BadgeCriteria.empty()` if the DB returns null. For `updateDefinition`, use `.update(params.toJson()).eq('id', id).eq('organisation_id', organisationId)` to ensure organisation scoping even on updates.

The `UpdateBadgeDefinitionParams` should use Dart's named optional parameters with no required fields (all nullable) so callers only send changed fields. Use `json_serializable` code generation if the project already uses it, otherwise manual fromJson/toJson is sufficient for these small models. Document the criteria JSON schema shape in the BadgeCriteria class docstring for future maintainers.

Unit tests with mocked SupabaseClient covering: (1) fetchAllDefinitions returns correctly mapped BadgeDefinition list with parsed BadgeCriteria, (2) BadgeCriteria.fromJson correctly parses both threshold=3 and threshold=15 variants including honorar_level field, (3) BadgeCriteria.toJson round-trips correctly (parse then serialise produces identical JSON), (4) createDefinition passes criteria as JSON to Supabase insert, (5) fetchDefinitionById returns null when Supabase returns empty list, (6) fetchAllDefinitions default call includes is_active filter. Integration test: create a definition with criteria, fetch it back, verify criteria fields match. Test BadgeCriteriaType enum exhaustiveness — all enum values must have corresponding fromJson handling. 90%+ coverage target.