Backend:
- Add /api/agent-group/categorize endpoint — AI groups agents by role
- Add /api/agent-group/filter endpoint — filter by selected groups
- Groups with default_enabled=false (advertiser, brand) are unchecked
Frontend:
- Add agent groups section in Step2EnvSetup.vue
- 'Auto-categorize' button triggers AI grouping
- Show groups with checkboxes (enabled groups checked, disabled unchecked)
- Auto-remove unchecked agents when proceeding to Step 3
- Show selected count summary
