#4202 Swamp Club collective GUI: show the swamp serve instances currently connected to a collective

Problem

A collective's page in the swamp.club GUI shows members, but not which swamp serve instances are running or connected under that collective. Operators can't tell from the GUI which serve instances are online, who started them, or when they last checked in. They have to ask teammates or SSH into hosts.

Proposed solution

If serve instances already register or heartbeat with swamp.club (for example for auth, collective admission, or the inbox check), surface that in the collective view with a "Serve instances" tab or panel. Per instance it would show:

• Instance name or ID, hostname, and swamp version
• Owner or identity that started it
• Connection state (connected / last seen timestamp)
• Optionally: repo or datastore identity, bound address, and connected worker count

If serve doesn't report to swamp.club today, add an opt-in lightweight registration/heartbeat (authenticated, collective-scoped, expiring after a missed heartbeat). It would need to be clear in the manual's outbound requests section, and could be turned off with a flag. Visibility should be limited to collective members, ideally with admin-only detail.

Alternatives considered

• A CLI-only command such as swamp serve list --collective <name>. It helps scripting but doesn't give the at-a-glance GUI view.
• Querying each serve instance directly. This requires knowing its address and having network reach, which defeats the purpose.

Open questions

• Does swamp.club currently have any per-instance signal from serve to build on?
• Privacy: which fields (hostname, address) should be exposed, and to whom?