feat(server)!: require a logged-in account for every channel request

Drop unauthenticated guests from channels: the allow_guests setting, the
/guests endpoints and the guest seat table are gone, and requests without an
account session get 401 account_required. Share links are unaffected.
Contract version is now 8.0.0.

contracts/api-contract.json

@@ -1,3 +1,3 @@
 {
-  "version": "7.0.0"
+  "version": "8.0.0"
 }