Make Proxy Server Retry Parameters Configurable

## Description

In https://github.com/aws/graph-explorer/blob/main/packages/graph-explorer-proxy-server/src/node-server.ts#L63-L64, it would be nice if the `retryDelay` and `refetchMaxRetries` options were configurable via environment variables. For large synchronizations over a slow connection, the `retryDelay` seems to act as an effective timeout without a way to increase it.

## Preferred Solution

Ability to control `retryDelay` and `refetchMaxRetries` via environment variables

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Start in packages/graph-explorer-proxy-server/src/node-server.ts around lines 63-64, where retryDelay and refetchMaxRetries are configured. Trace how the proxy server reads environment variables and confirm the expected value handling. Done means both retry parameters can be controlled through environment variables for slow or large synchronizations.

Written by the indexing model from the issue text.