Build Bufdir API integration service stub

Define `abstract class BufdirApiIntegrationService` with three async methods. Use Riverpod's `Provider` with a factory that reads the `BUFDIR_DELIVERY_MODE` env var (passed as a Dart const or via Supabase Edge Function environment) to select the implementation. The `BufdirFileDeliveryService` constructor should accept a `SupabaseClient` and a `BufdirAuditLogger` — both injected via Riverpod. The `BufdirDeliveryResult` class should be immutable (use `@immutable` annotation or a Dart record).

Document the future `BufdirDirectApiService` with a detailed `// TODO` block referencing the interface contract task (task-011) and listing the expected Bufdir API endpoint path, auth header format, and payload schema placeholder. Avoid using `dynamic` anywhere — all method signatures must be strongly typed. Register the provider in a dedicated `bufdir_providers.dart` file to keep provider definitions co-located with the service.

Unit tests (flutter_test with mocktail) must cover: (1) file delivery — mock Supabase Storage upload succeeds, assert `BufdirDeliveryResult` has status `pending_review` and non-empty `package_urls`; (2) file delivery — mock Supabase Storage upload throws, assert result has status `failed` and error message is populated; (3) configuration routing — with `BUFDIR_DELIVERY_MODE=fileDelivery`, assert `BufdirFileDeliveryService` is injected; (4) interface contract — instantiate `BufdirDirectApiService` and assert calling any method throws `UnimplementedError` with a descriptive message; (5) audit log write — assert `bufdir_export_audit_log` insert is called with correct fields on both success and failure paths. Riverpod provider override pattern must be demonstrated in at least one test to confirm testability. Test coverage for the stub implementation must be at least 90%.