Add brand analysis columns to onboarding database and migration scripts

backend/database/migrations/update_onboarding_user_id_to_string.sql

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+-- Migration: Update onboarding_sessions.user_id from INTEGER to STRING
+-- This migration updates the user_id column to support Clerk user IDs (strings)
+
+-- Step 1: Alter the user_id column type from INTEGER to VARCHAR(255)
+ALTER TABLE onboarding_sessions 
+ALTER COLUMN user_id TYPE VARCHAR(255);
+
+-- Step 2: Create an index on user_id for faster lookups
+CREATE INDEX IF NOT EXISTS idx_onboarding_sessions_user_id ON onboarding_sessions(user_id);
+
+-- Note: This migration assumes no existing data needs to be preserved
+-- If you have existing data with integer user_ids, you may need to:
+-- 1. Backup the data first
+-- 2. Clear the table or convert the integers to strings
+-- 3. Then apply this migration
+