5 tiers selectable in Sim Lab config panel. Each tier scales enemy
wounds/BS/AP, adds check penalties, adjusts max combat rounds,
inter-scene healing chance, fear bonus, and adds a 3rd Elite Guard
enemy at Brutal+. Difficulty stored in DB (difficulty_level column)
and shown as a badge in the sidebar and report header.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>