feat(time): 프로젝트 전체 시간 KST(UTC+9) 통일

- app/timezone.py 신설: KST 고정오프셋(+9, DST 없음) + now_kst/today_kst/to_kst_iso
- DB 직렬화 created_at/updated_at/decided_at/uploaded_at 표시를 UTC→KST 로 변환
  (expense/cupang/vacation db.py)
- _now_iso(JSON 저장) KST 기준
- 라우터 date.today()/datetime.now() → today_kst()/now_kst() (달력 '오늘'/엑셀 파일명)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/modules/vacation/db.py

@@ -22,6 +22,8 @@ from typing import Any
 from psycopg.rows import dict_row
 from psycopg_pool import ConnectionPool
 
+from app.timezone import KST
+
 from .store import EDITABLE_STATUSES, STATUSES, VACATION_TYPES, compute_days, normalize_half
 
 
@@ -449,7 +451,7 @@ class VacationDBStore:
         for k in ("created_at", "updated_at", "decided_at"):
             v = out.get(k)
             if isinstance(v, datetime):
-                out[k] = v.astimezone(timezone.utc).isoformat(timespec="seconds")
+                out[k] = v.astimezone(KST).isoformat(timespec="seconds")
         out["days"] = cls._num(out.get("days", 0))
         return out
 
@@ -466,7 +468,7 @@ class VacationDBStore:
         for k in ("created_at", "updated_at"):
             v = out.get(k)
             if isinstance(v, datetime):
-                out[k] = v.astimezone(timezone.utc).isoformat(timespec="seconds")
+                out[k] = v.astimezone(KST).isoformat(timespec="seconds")
         return out
 
     @classmethod
@@ -481,5 +483,5 @@ class VacationDBStore:
         for k in ("created_at", "updated_at"):
             v = out.get(k)
             if isinstance(v, datetime):
-                out[k] = v.astimezone(timezone.utc).isoformat(timespec="seconds")
+                out[k] = v.astimezone(KST).isoformat(timespec="seconds")
         return out

app/modules/vacation/router.py

@@ -11,7 +11,7 @@ from __future__ import annotations
 
 import calendar as _calendar
 import io
-from datetime import date, datetime
+from datetime import date
 from typing import Any
 
 from fastapi import APIRouter, Depends, Form, HTTPException, Request
@@ -22,6 +22,8 @@ from fastapi.responses import (
     StreamingResponse,
 )
 
+from app.timezone import now_kst, today_kst
+
 from .store import HALF_LABELS, HALVES, VACATION_TYPES
 
 router = APIRouter(prefix="/vacation", tags=["vacation"])
@@ -107,7 +109,7 @@ def _guard(
 
 
 def _ym(request: Request) -> tuple[int, int]:
-    today = date.today()
+    today = today_kst()
     try:
         year = int(request.query_params.get("year") or today.year)
         month = int(request.query_params.get("month") or today.month)
@@ -170,7 +172,7 @@ def _build_calendar(
         date_from=range_start.isoformat(), date_to=range_end.isoformat()
     )
 
-    today = date.today()
+    today = today_kst()
     weeks: list[dict[str, Any]] = []
     for week in weeks_dates:
         w_start, w_end = week[0], week[-1]
@@ -234,7 +236,7 @@ async def index(request: Request) -> HTMLResponse:
 
     year, month = _ym(request)
     sel = request.query_params.get("date") or ""
-    today = date.today()
+    today = today_kst()
     if not sel:
         sel = (
             today.isoformat()
@@ -315,7 +317,7 @@ async def new_form(request: Request) -> HTMLResponse:
             "page_subtitle": "휴가 종류 · 기간 · 사유 입력",
             "mode": "new",
             "req": None,
-            "default_date": date.today().isoformat(),
+            "default_date": today_kst().isoformat(),
         }
     )
     return render_template(request, "vacation/form.html", ctx)
@@ -419,7 +421,7 @@ async def settings_page(request: Request) -> HTMLResponse:
             {"reason": "휴가 설정은 관리자만 접근할 수 있습니다.", "is_admin": False},
             status_code=403,
         )
-    today = date.today()
+    today = today_kst()
     try:
         year = int(request.query_params.get("year") or today.year)
     except ValueError:
@@ -551,7 +553,7 @@ async def export_xlsx(
     if store is None:
         raise HTTPException(status_code=503, detail="vacation_db 미설정")
 
-    today = date.today()
+    today = today_kst()
     try:
         year = int(request.query_params.get("year") or today.year)
         month = int(request.query_params.get("month") or 0)
@@ -599,7 +601,7 @@ async def export_xlsx(
     wb.save(buf)
     buf.seek(0)
     scope = f"{year}{('%02d' % month) if (1 <= month <= 12) else ''}"
-    fname = f"vacation_{scope}_{datetime.now().strftime('%Y%m%d_%H%M%S')}.xlsx"
+    fname = f"vacation_{scope}_{now_kst().strftime('%Y%m%d_%H%M%S')}.xlsx"
     return StreamingResponse(
         buf,
         media_type="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",