BREAKING CHANGE: Removed core_companies, core_users, core_user_roles tables
Migrations:
- Create 020_unify_schema.sql: adds tenant_id, email, name to users table
- Create user_roles table (replaces core_user_roles)
- Disable 009_create_core_tables.sql (renamed to .disabled)
- Update 010_seed_super_admin.sql to use unified tables
Backend Repositories:
- company_repository.go: use companies table with INT id
- user_repository.go: use users/user_roles with INT id conversion
Seeders:
- All seeders now use companies/users/user_roles tables
- Removed all core_* table insertions
- Query companies by slug to get SERIAL id
This eliminates the redundancy between core_* and legacy tables.
