Wire state layer to ReportHistoryService

If using BLoC: define events as a sealed class (FetchReportHistory, FilterChanged, LoadNextPage, ReexportReport, RedownloadReport) and use `EventTransformer.droppable()` for LoadNextPage and `EventTransformer.restartable()` for filter changes to handle in-flight cancellation. If using Riverpod: use `AsyncNotifier` with an `updateFilter` method and a `loadNextPage` method guarded by an `_isFetching` flag. Model per-record re-export loading state as a `Map` inside the loaded state rather than a separate state class to avoid overwriting the list. Emit side-effects (snackbar messages, URL-open events) via a separate `Stream` or a Riverpod `ref.listenSelf()` channel so they do not interfere with the primary state stream.

Dispose the state layer at route pop to clear sensitive report data from memory.

Unit tests using flutter_test + mocktail: test that initial state is ReportHistoryInitial; test that after construction the first fetch is triggered and state transitions to ReportHistoryLoading then ReportHistoryLoaded; test that a filter change resets to page 1 and re-fetches; test that LoadNextPage appends records and updates hasNextPage correctly; test that LoadNextPage is ignored when already loading; test that service error on initial fetch transitions to ReportHistoryError; test that re-export success emits a SnackbarMessage side-effect without leaving loaded state; test that re-export error emits an error SnackbarMessage. All tests use mocked service/coordinator with pre-defined responses.