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- serializer: enrich techniques with tactics from service at serialize time (graceful empty tactics if bundle outdated)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/models/simulation.py

@@ -25,8 +25,7 @@ class Simulation(db.Model):  # type: ignore[name-defined]
         index=True,
     )
     name = db.Column(db.String(255), nullable=False)
-    mitre_technique_id = db.Column(db.String(32), nullable=True)
-    mitre_technique_name = db.Column(db.String(255), nullable=True)
+    techniques = db.Column(db.JSON, nullable=False, default=list)
     description = db.Column(db.Text, nullable=True)
     commands = db.Column(db.Text, nullable=True)
     prerequisites = db.Column(db.Text, nullable=True)