perf(events): list_events total 을 DB COUNT 푸시다운 (R10)

전체 Event.id 를 메모리 로딩 후 len() 하던 것을 select(func.count(Event.id)) 로 전환 —
행 수에 선형이던 메모리/전송 비용 제거. 결과 동등(단순 카운트라 golden-diff 불요).

검증: py_compile 통과.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/api/events.py

@@ -21,7 +21,7 @@ from zoneinfo import ZoneInfo
 
 from fastapi import APIRouter, Body, Depends, HTTPException, Query
 from pydantic import BaseModel, Field
-from sqlalchemy import and_, or_, select
+from sqlalchemy import and_, func, or_, select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from core.auth import get_current_user
@@ -388,10 +388,10 @@ async def list_events(
         )
 
     base = select(Event).where(and_(*where))
-    total_q = await session.execute(
-        select(Event.id).where(and_(*where))
-    )
-    total = len(total_q.scalars().all())
+    # R10: 전체 ID 로딩 후 len() 대신 DB COUNT 푸시다운 (행 수 선형 메모리/전송 비용 제거).
+    total = (
+        await session.execute(select(func.count(Event.id)).where(and_(*where)))
+    ).scalar() or 0
 
     rows = await session.execute(
         base.order_by(Event.created_at.desc())