Address issue where users cannot be assigned to groups in production

Add error handling to the user group assignment to gracefully handle missing `created_at` column in production environments, and document the fix.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: a8a78c07-e900-425c-a577-5b4c5894379d
Replit-Commit-Checkpoint-Type: full_checkpoint
Replit-Commit-Screenshot-Url: https://storage.googleapis.com/screenshot-production-us-central1/1957c339-2757-4d1f-8e92-e9f71a1ce58e/a8a78c07-e900-425c-a577-5b4c5894379d/yj7Yt4E

fix-production-usergroups.sql

@@ -0,0 +1,9 @@
+-- Fix production user_groups table to add missing created_at column
+-- This should be run on your production database
+
+-- Add the missing created_at column to user_groups table
+ALTER TABLE user_groups 
+ADD COLUMN IF NOT EXISTS created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP;
+
+-- Verify the table structure
+\d user_groups;