feat(dashboard): Phase 13 — Executive Dashboard

PostureSnapshot model, Alembic migration (b039exec), schemas, service
aggregating all phases (coverage/risk/operations/knowledge/MTTD), and
router at /api/v1/dashboard with executive view, KPIs, coverage-by-tactic,
posture-history, posture-snapshot, and activity-feed endpoints.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/main.py

@@ -43,6 +43,7 @@ from app.routers import ownership as ownership_router
 from app.routers import attack_paths as attack_paths_router
 from app.routers import knowledge as knowledge_router
 from app.routers import risk_intelligence as risk_router
+from app.routers import executive_dashboard as dashboard_router
 from app.domain.errors import DomainError
 from app.middleware.error_handler import domain_exception_handler
 from app.middleware.request_context import RequestContextMiddleware
@@ -145,6 +146,7 @@ app.include_router(ownership_router.router, prefix="/api/v1")
 app.include_router(attack_paths_router.router, prefix="/api/v1")
 app.include_router(knowledge_router.router, prefix="/api/v1")
 app.include_router(risk_router.router, prefix="/api/v1")
+app.include_router(dashboard_router.router, prefix="/api/v1")
 
 
 @app.get("/health", include_in_schema=False)