feat(backend): sprint 3 — multi-technique simulations + MITRE matrix

- Simulation model: replace mitre_technique_id/name scalars with techniques JSON column [{id, name}]
- Alembic migration 0003: add techniques, backfill from scalars, drop old columns (reversible)
- MITRE service: add get_tactics(), lookup_name(), get_matrix() with canonical tactic order and sub-technique nesting
- serializer: enrich techniques with tactics from service at serialize time (graceful empty tactics if bundle outdated)
- simulation_workflow: PATCH now accepts technique_ids list, validates against bundle, deduplicates preserving order, auto-transitions on non-empty list
- simulations API: add GET /api/mitre/matrix endpoint (503 if bundle absent)
- test_mitre.py: updated _reset_mitre fixture, added T1059.006 sub-technique, 14 new tests for get_tactics/lookup_name/get_matrix/matrix endpoint
- test_simulations_techniques.py: 20 new tests covering AC-13.1 to AC-13.5 (create, PATCH, dedup, auto-transition, SOC blocked, migration backfill logic)

Total: 161 tests passing. ruff clean. mypy: no new errors.

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

backend/app/serializers.py

@@ -20,13 +20,22 @@ def serialize_user_brief(user: User) -> dict[str, Any]:
     return {"id": user.id, "username": user.username}
 
 
+def _enrich_techniques(raw: list[dict[str, Any]]) -> list[dict[str, Any]]:
+    """Attach tactics to each {id, name} snapshot from the MITRE service."""
+    from backend.app.services import mitre as mitre_svc
+
+    return [
+        {"id": t["id"], "name": t["name"], "tactics": mitre_svc.get_tactics(t["id"])}
+        for t in (raw or [])
+    ]
+
+
 def serialize_simulation(simulation: Simulation) -> dict[str, Any]:
     return {
         "id": simulation.id,
         "engagement_id": simulation.engagement_id,
         "name": simulation.name,
-        "mitre_technique_id": simulation.mitre_technique_id,
-        "mitre_technique_name": simulation.mitre_technique_name,
+        "techniques": _enrich_techniques(simulation.techniques or []),
         "description": simulation.description,
         "commands": simulation.commands,
         "prerequisites": simulation.prerequisites,