fix(backend): sprint 3 post-review — migration nullable + dead code + tactic names + tests

- Migration 0003: enforce techniques NOT NULL via batch_alter_table (AC-13.1 DDL spec)
- Migration 0003: remove unused _sims table proxy and orphaned column/table imports
- mitre.py: rename _TACTIC_NAMES → TACTIC_NAMES (public); add all 12 correct display names
- mitre.py: use TACTIC_NAMES dict in _build_matrix() to fix "Command And Control" → "Command and Control"
- test_mitre.py: add T1071 fixture entry under command-and-control; assert tactic_name lowercase "and"
- test_simulations_techniques.py: real Alembic round-trip test asserting techniques NOT NULL after upgrade

backend/app/services/mitre.py

@@ -26,7 +26,7 @@ _TACTIC_ORDER = [
     "impact",
 ]
 
-_TACTIC_NAMES: dict[str, str] = {
+TACTIC_NAMES: dict[str, str] = {
     "initial-access": "Initial Access",
     "execution": "Execution",
     "persistence": "Persistence",
@@ -102,7 +102,7 @@ def _build_matrix(entries: list[dict[str, Any]]) -> list[dict[str, Any]]:
         techs = tactic_techs.get(tactic_id, [])
         # Sort techniques alphabetically.
         techs_sorted = sorted(techs, key=lambda x: x["name"])
-        tactic_name = _TACTIC_NAMES.get(tactic_id, tactic_id.replace("-", " ").title())
+        tactic_name = TACTIC_NAMES.get(tactic_id, tactic_id.replace("-", " ").title())
         matrix.append(
             {
                 "tactic_id": tactic_id,