Add retry logic with exponential backoff to backend API client

[Jayy4rl]

Context: The backend client (Issue 382) makes HTTP calls that can fail transiently. Automatic retries improve reliability.

Requirements:

• Add retries option (default 3) to createBackendClient.
• Retry on 429, 502, 503, 504 and network errors. Use exponential backoff: 100ms × 2^attempt (cap at 5s).

Acceptance criteria:

• A mocked 503 response triggers 3 retries before failing.
• A 404 is not retried.