fix: address PR #177 review — move import os to top-level, add audit idempotency guard

- Move `import os` from inside evaluate_game() to module top-level imports
  (lazy imports are only for circular dependency avoidance)
- Add get_or_none idempotency guard before RefractorBoostAudit.create()
  inside db.atomic() to prevent IntegrityError on UNIQUE(card_state, tier)
  constraint in PostgreSQL when apply_tier_boost is called twice for the
  same tier
- Update atomicity test stub to provide card_state/tier attributes for
  the new Peewee expression in the idempotency guard

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/routers_v2/refractor.py

@@ -1,3 +1,5 @@
+import os
+
 from fastapi import APIRouter, Depends, HTTPException, Query
 import logging
 from typing import Optional
@@ -307,8 +309,6 @@ async def evaluate_game(game_id: int, token: str = Depends(oauth2_scheme)):
         logging.warning("Bad Token: [REDACTED]")
         raise HTTPException(status_code=401, detail="Unauthorized")
 
-    import os
-
     from ..db_engine import RefractorCardState, Player, StratPlay
     from ..services.refractor_boost import apply_tier_boost
     from ..services.refractor_evaluator import evaluate_card

app/services/refractor_boost.py

@@ -667,7 +667,11 @@ def apply_tier_boost(
             audit_data["battingcard"] = new_card.id
         else:
             audit_data["pitchingcard"] = new_card.id
-        _audit_model.create(**audit_data)
+        existing_audit = _audit_model.get_or_none(
+            (_audit_model.card_state == card_state.id) & (_audit_model.tier == new_tier)
+        )
+        if existing_audit is None:
+            _audit_model.create(**audit_data)
 
         # 8b. Update RefractorCardState — this is the SOLE tier write on tier-up.
         card_state.current_tier = new_tier

tests/test_refractor_boost_integration.py

@@ -1115,7 +1115,19 @@ class TestAtomicity:
         """
 
         class _FailingAuditModel:
-            """Stub that raises on .create() to simulate audit write failure."""
+            """Stub that raises on .create() to simulate audit write failure.
+
+            Provides card_state/tier attributes for the Peewee expression in the
+            idempotency guard, and get_or_none returns None so it proceeds to
+            create(), which then raises to simulate the failure.
+            """
+
+            card_state = RefractorBoostAudit.card_state
+            tier = RefractorBoostAudit.tier
+
+            @staticmethod
+            def get_or_none(*args, **kwargs):
+                return None
 
             @staticmethod
             def create(**kwargs):