Implement full export flow with audit record persistence

Structure requestFinalExport() in three distinct phases: (A) edge function invocation via invokeEdgeFunction(), (B) file generation via the routing layer, (C) atomic persistence. For phase C, prefer a Supabase RPC function that inserts the audit record and registers the file in a single PostgreSQL transaction rather than two sequential client calls — this is the only reliable way to achieve atomicity from a mobile client. If an RPC is not yet available, implement compensating logic: if audit insert fails, call supabase.storage.from('bufdir-exports').remove([filePath]) before rethrowing. The BufdirExportResult should be a plain Dart data class (freezed recommended) for immutability.

Never store signed URLs in the audit record — store the storage path and generate signed URLs on demand.

Unit tests with flutter_test: (1) successful export inserts exactly one audit record and returns BufdirExportResult, (2) file generator failure (throws) results in zero audit record inserts (assert mock repository has zero calls), (3) audit repository throws on insert — assert storage cleanup is called (deleteFile mock invoked once), (4) returned BufdirExportResult fields match expected values, (5) user ID in audit record matches authenticated user from Supabase session. Integration test (if Supabase test instance available): verify atomicity by injecting a failure after file upload and before audit insert, confirm no orphaned storage objects.