feat(backend): sprint 5 — SimulationTemplate CRUD + instantiation

- SimulationTemplate model + migration 0005 (CREATE TABLE + name index)
- 5 CRUD endpoints under /api/templates (admin|redteam only, SOC 403)
- POST /api/engagements/<eid>/simulations extended with optional template_id
- serialize_template() reusing _enrich_techniques/_enrich_tactics helpers
- IntegrityError → 409 for duplicate name on both POST and PATCH
- 28 new tests (CRUD, RBAC, dedup, instantiation, migration round-trip)
- 221 tests pass; ruff clean; mypy clean

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

backend/app/models/__init__.py

@@ -1,6 +1,15 @@
 """SQLAlchemy models."""
 from backend.app.models.engagement import Engagement, EngagementStatus
 from backend.app.models.simulation import Simulation, SimulationStatus
+from backend.app.models.simulation_template import SimulationTemplate
 from backend.app.models.user import User, UserRole
 
-__all__ = ["User", "UserRole", "Engagement", "EngagementStatus", "Simulation", "SimulationStatus"]
+__all__ = [
+    "User",
+    "UserRole",
+    "Engagement",
+    "EngagementStatus",
+    "Simulation",
+    "SimulationStatus",
+    "SimulationTemplate",
+]