Implement orchestrator step sequencing

Structure the orchestrator as a sequence of private async methods (_createAudit, _queryActivities, _mapColumns, _generateFile, _bundleAttachments, _storeAndFinalize) each called in order from a single public runExport method. Wrap each private method call in a try/catch that calls _markFailed(auditId, e.toString()) before rethrowing — this keeps the happy-path code linear and readable. Use Dart's sealed classes (Dart 3+) for ExportResult to force exhaustive handling at the call site. Run _generateFile inside compute() passing a plain data record (avoid passing closures).

The BufdirExportOrchestratorService should be registered as a Riverpod AsyncNotifierProvider so the UI can watch export progress via audit record polling. Keep the orchestrator thin: it should contain sequencing logic only, never business rules about columns or activity filtering.

Unit tests (flutter_test): use mock implementations of BufdirActivityQueryService, BufdirColumnMapper, file generator, and BufdirExportAuditService to verify correct step sequencing via call-order assertions. Test the zero-row path returns ExportSuccess with rowCount=0 and null downloadUrl. Test that audit status transitions occur in the correct order using a recorded fake. Test that includeAttachments=false skips the bundling step entirely.

Integration test (Supabase local emulator): run a full pipeline against real Supabase tables with seed data for NHF, Blindeforbundet, and HLF to confirm the audit record reaches 'completed' status and the signed URL is accessible. Target ≥85% line coverage on the orchestrator class.