docs: use valid env example placeholders

[Draxilion]

Summary

• Replace the malformed OAuth callback placeholder with the local callback URL documented in the README
• Use an empty optional token address placeholder instead of a non-address sentinel
• Use numeric token decimals so copied env files satisfy config validation shape

Test Plan

• Validated URL-shaped env placeholders with Python urlparse ✅
• Validated token decimals parse as a non-negative integer ✅

Closes #2

[vercel]

@feminefa is attempting to deploy a commit to the Bytegum Team on Vercel.

A member of the Team first needs to authorize it.