feat: add last active time per user

scripts/migrate/016_user_activity.sql

@@ -0,0 +1,7 @@
+-- +migrate Up
+
+-- 2023-05-02: Add a last_active column to users, updated whenever the user modifies their profile or members.
+-- This is not directly exposed in the API.
+-- Potential future use cases: showing total number of active users, pruning completely empty users if they don't log in?
+
+alter table users add column last_active timestamptz not null default now();