Implement payment retry logic

[mftee]

Adds a retry package with a configurable Policy for automatically retrying failed payment operations with a backoff delay between attempts.

Closes #58

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[AbdulmujibOladayo]

Good retry primitive — Policy.Do stops immediately on success, respects ctx cancellation during backoff instead of blocking past a cancelled context, and returns the last real error rather than a generic timeout. DefaultPolicy's 3 attempts / 2s backoff is a sensible default for payment retries.