fix: timestamp fallback에서 UTC→KST 변환 추가

Codex 리뷰 지적: timestamp fallback 분기에서 UTC aware 값을
replace(tzinfo=None)로 tz만 제거하면 KST time_bucket과 9시간 어긋남.
astimezone(KST) 후 tz 제거하도록 수정.

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

prediction/cache/vessel_store.py

@@ -1,8 +1,11 @@
 import logging
 from datetime import datetime, timezone
 from typing import Optional
+from zoneinfo import ZoneInfo
 
 import numpy as np
+
+_KST = ZoneInfo('Asia/Seoul')
 import pandas as pd
 
 logger = logging.getLogger(__name__)
@@ -126,8 +129,9 @@ class VesselStore:
             max_ts = pd.to_datetime(df_all['timestamp'].dropna()).max()
             if hasattr(max_ts, 'to_pydatetime'):
                 max_ts = max_ts.to_pydatetime()
+            # timestamp는 UTC aware → KST wall-clock naive로 변환
             if isinstance(max_ts, datetime) and max_ts.tzinfo is not None:
-                max_ts = max_ts.replace(tzinfo=None)
+                max_ts = max_ts.astimezone(_KST).replace(tzinfo=None)
             self._last_bucket = max_ts
 
         vessel_count = len(self._tracks)