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.

server/src/WebDrop.Api/Endpoints.Accounts.cs

@@ -101,7 +101,6 @@ public static partial class Endpoints
         {
             if (!TryAccount(ctx, accounts, out var username)) throw new WebDropException(ErrorCode.AccountRequired);
             accounts.RevokeDevice(username, id);
-            svc.ReleaseAccountDevice(username, id);
             return Results.NoContent();
         });