Add SQL migration 100_user_weight_profiles_default_seed.sql with system
presets Эконом/Комфорт/Бизнес (user_id='__system__'). Migration is
idempotent via ON CONFLICT DO UPDATE.
Backend:
- weight_profiles.py: add SYSTEM_USER_ID constant + list_profiles_with_system()
- admin_weight_profiles.py: add include_system query param to GET list endpoint
Tests: 3 new tests covering include_system flag and service sentinel behaviour.
