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/api/__init__.py

@@ -2,6 +2,7 @@
 from backend.app.api.auth import auth_bp
 from backend.app.api.engagements import engagements_bp
 from backend.app.api.simulations import simulations_bp
+from backend.app.api.templates import templates_bp
 from backend.app.api.users import users_bp
 
-__all__ = ["auth_bp", "users_bp", "engagements_bp", "simulations_bp"]
+__all__ = ["auth_bp", "users_bp", "engagements_bp", "simulations_bp", "templates_bp"]