V3: shared Effectiveness/Regulatory engines + Executive Intelligence modules

- Add EffectivenessStatus and RegulatoryStatus domain enums
- Add Effectiveness/Regulatory/ExecutiveSnapshot models and services
- Add EvidencePackService and SnapshotService
- Add effectiveness/regulatory/executive_snapshot migrations
- Make audit_events.organization_id nullable
- Surface new modules on landing page (Welcome.tsx)
- Add V3 BRDs and IMPLEMENTATION_PLAN_V3.md
- Add Effectiveness/Regulatory/SharedEngines tests
