Store the person uuid the master issues, so renames keep our identity

The welcome now carries a personId; it lands in the instance_id setting
and rides on every hello and HTTP report from then on. The master keys
the person by that uuid, so changing instance_name moves our history to
the new name instead of starting a stranger. Read-only in Settings on
purpose — editing it by hand would detach this machine from its person.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/api/settings/route.ts

@@ -9,6 +9,9 @@ export async function GET() {
     for (const key of VALID_KEYS) {
         result[key] = getSetting(key);
     }
+    // Read-only on purpose (not in VALID_KEYS): the master issues this uuid,
+    // and editing it by hand would detach the instance from its person.
+    result.instance_id = getSetting('instance_id');
     return NextResponse.json(result);
 }