fix: batch account hydration to prevent 429 rate limits

[ndycode]

fixes a vulnerability in index.ts where Promise.all allowed unbounded concurrent requests to openai's auth0 endpoints during account hydration.

changes:

• replaced the unbounded Promise.all over all stored accounts with a batched concurrency pool.
• set chunk size to 3 to prevent instant 429 rate limit blocks when hydrating >10 accounts.

this touches the startup hydration path. isolated in a clean branch and ran npm run build && npm run test. everything is green locally.

let me know if you need changes to the implementation.