Implement non-blocking upload with progress streaming

Flutter isolates cannot share objects — you must pass the compressed Uint8List and auth token as primitive-compatible messages to the isolate via SendPort/ReceivePort. The Supabase Flutter SDK's storage uploadBinary method accepts an onUploadProgress callback — call this from the isolate and send progress integers back via the ReceivePort. For retry logic, implement a RetryPolicy value object with maxAttempts and baseDelayMs; the exponential delay is baseDelayMs * 2^(attempt-1). Distinguish transient errors (SocketException, TimeoutException, HTTP 503/429) from permanent ones (HTTP 400/401/403/404) before deciding to retry.

For cancellation, use an IsolateCancel flag: the isolate checks it between retries and during the progress callback; on cancel, it deletes the partial upload and sends a cancelled message. Note that Supabase Storage does not support true resumable uploads via the Flutter SDK as of 2025 — cancellation deletes and the next call starts fresh. Expose cancelUpload(claimId) from the service class, keyed by claimId so concurrent uploads can be cancelled independently.

Unit tests (flutter_test): mock the Supabase Storage client within the isolate by injecting a factory. Test retry logic: simulate SocketException on attempt 1 and 2, success on attempt 3 — assert three upload calls and correct backoff delays using fake async. Test exhausted retries: three consecutive SocketExceptions — assert UploadProgress.failed. Test cancellation: cancel mid-upload — assert cancelled event and StorageRepository.delete called.

Test permanent error (403): assert no retry, immediate failed. Integration test: upload a real 400 KB JPEG to a Supabase test bucket via the service, assert progress events received, file present in bucket, and claim_receipts row linked. Use flutter_test's FakeAsync to test backoff timing without real delays.