chore: relocate backend into apps/api ahead of monorepo merge

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

apps/api/utils/logActivity.util.js

@@ -0,0 +1,26 @@
+const mdl_UserActivity = require('../models/users/user_activity.mdl');
+
+/**
+ * Fire-and-forget activity logger. Never throws — failures are console-logged only.
+ *
+ * @param {number}  userId
+ * @param {string}  action      — e.g. 'login', 'submit_task', 'deactivate_user'
+ * @param {Object}  [opts]
+ * @param {string}  [opts.entityType] — e.g. 'user', 'task', 'lesson'
+ * @param {number}  [opts.entityId]
+ * @param {number}  [opts.sessionId]
+ * @param {Object}  [opts.details]    — free-form JSONB context
+ */
+function logActivity(userId, action, { entityType = null, entityId = null, sessionId = null, details = null } = {}) {
+  const numericId = entityId !== null ? Number(entityId) : null;
+  mdl_UserActivity.create({
+    user_id:     userId,
+    action,
+    entity_type: entityType,
+    entity_id:   Number.isFinite(numericId) ? numericId : null,
+    session_id:  sessionId,
+    details,
+  }).catch((err) => console.error('[USER_ACTIVITY] Failed to log:', action, err.message));
+}
+
+module.exports = logActivity;