[FIX] Smart Invoices migration - category mapping & deduplication

Problem / Проблема

• Smart invoices: SOURCE=50, TARGET=48 (2 LOST!)
• Category duplicates: 6 duplicates instead of 1
• Mapping empty: smart_category_31 in SQLite is empty
• Files not migrated

Solution / Решение

1. Fix category mapping in migrateCategoriesBySmart()
2. Add duplicate cleanup before migration
3. Add STAGE_ID validation
4. Implement file migration for smart_invoices

Files to change

• src/Commands/MigrateCommand.php
• src/Utils/EntityTransformer.php

Success criteria

• SOURCE count = TARGET count (50 = 50)
• 1 category on TARGET (not 6 duplicates)
• Mapping saved in SQLite
• STAGE_ID transforms correctly