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/models/__init__.py

@@ -40,6 +40,7 @@ from app.models.attack_path import (
 )
 from app.models.knowledge import Playbook, PlaybookVersion, LessonLearned
 from app.models.risk_intelligence import TechniqueRiskProfile
+from app.models.executive_dashboard import PostureSnapshot
 
 __all__ = [
     "User", "Technique", "Test", "TestTemplate", "Evidence",
@@ -63,4 +64,5 @@ __all__ = [
     "ExecutionStatus", "StepResultStatus", "TimelineActorSide", "TimelineEntryType",
     "Playbook", "PlaybookVersion", "LessonLearned",
     "TechniqueRiskProfile",
+    "PostureSnapshot",
 ]