Add 139 tests across 3 test files for the billing service layer:
- test_subscription_service.py (37 tests): tier lookup, subscription CRUD, upgrades, cancellation
- test_admin_subscription_service.py (39 tests): admin tier/subscription management, stats, billing history
- test_billing_service.py (43 tests): rewritten with correct fixtures after store→merchant migration
Fix production bug in deactivate_tier() — BusinessLogicException was missing
required error_code argument, now uses TIER_HAS_ACTIVE_SUBSCRIPTIONS.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>