Implement HTTP submission and response handling

Implement BufdirApiClient as a class injected with an http.Client (or Dio instance) so tests can substitute a mock. Use a RetryPolicy helper with configurable maxAttempts and backoff strategy — do not inline retry logic in the submit method. Route the actual HTTP call through a Supabase Edge Function (`functions/bufdir-submit`) which holds the real Bufdir credentials; the mobile client calls the Edge Function, not Bufdir directly. Use Dio's interceptor stack for logging (masked) and for injecting the Supabase JWT.

Model BufdirSubmissionResult with freezed sealed classes: `BufdirSubmissionResult` → `BufdirSubmissionSuccess(referenceNumber: String)` | `BufdirSubmissionError(type: BufdirErrorType, message: String, attempts: int)`. After each failed attempt, write an interim 'submission_failed' row to bufdir_export_audit_log so coordinators can see retry history.

Unit tests using flutter_test with a MockHttpClient (or mockito): (1) successful submission extracts reference number, (2) each error code (400, 409, 422, 500) maps to correct error variant, (3) three 500 errors exhaust retries and return transientFailure, (4) 409 is not retried, (5) network timeout surfaces as timeout error, (6) concurrent duplicate call returns same Future. Integration test against Bufdir sandbox environment (if available) covering happy-path and 409 conflict. All tests must run without real network access in CI.