Implement Accounting Exporter Service orchestrator

Use a sealed class or enum for ExportStatus (PENDING, COMPLETED, FAILED, DUPLICATE, EMPTY) to make exhaustive switch handling compile-time safe in Dart. Wrap the entire pipeline from 'create ExportRun' through 'update ExportRun to completed' in a try/catch/finally where the finally block guarantees ExportRun is never left in PENDING state. The claims update and ExportRun completion update should be issued as two sequential Supabase calls inside a Postgres transaction if the Supabase client supports it; otherwise use a Postgres function called via rpc() to keep atomicity server-side. Inject a Clock abstraction rather than calling DateTime.now() directly to keep tests deterministic.

The routing logic should delegate to an AccountingExporterFactory that reads OrgAccountingConfig — keep AccountingExporterService free of any Xledger/Dynamics-specific imports. Use Dart's async/await with structured error handling (no unawaited futures). Log export start, completion, and failure at INFO level with orgId and dateRange (no claim content).

Unit tests (flutter_test / dart test): mock all five injected dependencies and verify each orchestration branch independently — happy path Xledger, happy path Dynamics, DUPLICATE guard abort, empty claims result, exporter throws exception (verify ExportRun set to failed), claims update called with correct IDs. Integration test (Supabase local emulator): seed approved claims for an org, call triggerExport, assert ExportRun row in DB has status='completed' and all seeded claims have exported_at set. Regression test: re-run triggerExport for same org/dateRange, assert DUPLICATE result and no new ExportRun row created. Test coverage gate: 90% line coverage on AccountingExporterService class.