feat(server): Configure background relays without restarting#94

What Changed

Background relay settings live in one atomic, versioned server secret, managed by new t3 connect background-relay commands that read the credential from stdin and print redacted status. The server watches configuration and activity sharing changes on their own instead of only when publishing, and resyncs current activity until the relay acknowledges each thread for the current destination revision, including after ordinary publish retries run out.

Why

With #16618 background-relay-configuration, a background relay is configured through environment variables, so changing it means restarting the server and the credential sits in the process environment. This lets a host enroll or rotate its background relay while T3 Code keeps running.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes