feat: 관리자 대시보드 — 토큰·비용·응답시간 집계 API + 앱 /admin 페이지

- backend apps/stats: GET /api/v1/admin/stats?from&to → totals / byUser / byDay. ChatMessage(assistant) 집계만, is_superuser 아니면 403. 테스트 3개
- frontend features/admin: 요약 카드 4개(요청·토큰·비용·평균 응답), 일별 토큰 막대(div), 사용자별 표, 기간 프리셋(오늘/7/30/90일). 헤더에 ADMIN 만 아이콘. 테스트 2개

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

5_django_backend/apps/stats/apps.py

@@ -0,0 +1,6 @@
+from django.apps import AppConfig
+
+
+class StatsConfig(AppConfig):
+    name = "apps.stats"
+    verbose_name = "관리자 통계(사용량·비용·시간)"

5_django_backend/apps/stats/urls.py

@@ -0,0 +1,7 @@
+from django.urls import path
+
+from .views import AdminStatsView
+
+urlpatterns = [
+    path("stats", AdminStatsView.as_view()),
+]

5_django_backend/apps/stats/views.py

@@ -0,0 +1,106 @@
+"""관리자 대시보드 집계 — ChatMessage(assistant 행)에 이미 쌓인 토큰·비용·시간을 모아서 줌.
+
+  GET /api/v1/admin/stats?from=YYYY-MM-DD&to=YYYY-MM-DD
+    → { totals, byUser[], byDay[] }   (is_superuser 만. 아니면 403 FORBIDDEN)
+
+새로 모으는 건 없음. 기간 기본은 최근 30일(to 포함).
+"""
+
+from datetime import date, datetime, time, timedelta
+
+from django.db.models import Avg, Count, F, Sum
+from django.db.models.functions import TruncDate
+from django.utils import timezone
+from rest_framework.response import Response
+from rest_framework.views import APIView
+
+from apps.chat.models import ChatMessage
+from common.envelope import CodedError
+
+MAX_DAYS = 366
+
+
+def _parse_range(request) -> tuple[date, date]:
+    today = timezone.localdate()
+    raw_from, raw_to = request.query_params.get("from"), request.query_params.get("to")
+    try:
+        d_to = date.fromisoformat(raw_to) if raw_to else today
+        d_from = date.fromisoformat(raw_from) if raw_from else d_to - timedelta(days=29)
+    except ValueError:
+        raise CodedError(400, "VALIDATION_ERROR", "from/to 는 YYYY-MM-DD 여야 해")
+    if d_from > d_to:
+        raise CodedError(400, "VALIDATION_ERROR", "from 이 to 보다 늦어")
+    if (d_to - d_from).days >= MAX_DAYS:
+        raise CodedError(400, "VALIDATION_ERROR", f"기간은 최대 {MAX_DAYS}일")
+    return d_from, d_to
+
+
+def _agg(qs):
+    """공통 집계 — 요청 수·토큰·비용·평균 응답시간."""
+    return qs.aggregate(
+        requests=Count("id"),
+        input_tokens=Sum("input_tokens"),
+        output_tokens=Sum("output_tokens"),
+        cost_usd=Sum("cost_usd"),
+        avg_elapsed_ms=Avg("elapsed_ms"),
+    )
+
+
+def _row(d: dict) -> dict:
+    return {
+        "requests": d["requests"] or 0,
+        "inputTokens": d["input_tokens"] or 0,
+        "outputTokens": d["output_tokens"] or 0,
+        "totalTokens": (d["input_tokens"] or 0) + (d["output_tokens"] or 0),
+        "costUsd": round(d["cost_usd"] or 0.0, 6),
+        "avgElapsedMs": round(d["avg_elapsed_ms"]) if d["avg_elapsed_ms"] is not None else None,
+    }
+
+
+class AdminStatsView(APIView):
+    def get(self, request):
+        if not request.user.is_superuser:
+            raise CodedError(403, "FORBIDDEN", "관리자만 볼 수 있어")
+        d_from, d_to = _parse_range(request)
+        tz = timezone.get_current_timezone()
+        start = datetime.combine(d_from, time.min, tzinfo=tz)
+        end = datetime.combine(d_to + timedelta(days=1), time.min, tzinfo=tz)
+        base = ChatMessage.objects.filter(role="assistant", created_at__gte=start, created_at__lt=end)
+
+        by_user = (
+            base.values(user_id=F("session__user_id"), email=F("session__user__email"), name=F("session__user__user_name"))
+            .annotate(
+                requests=Count("id"),
+                input_tokens=Sum("input_tokens"),
+                output_tokens=Sum("output_tokens"),
+                cost_usd=Sum("cost_usd"),
+                avg_elapsed_ms=Avg("elapsed_ms"),
+                sessions=Count("session_id", distinct=True),
+            )
+            .order_by("-cost_usd", "-requests")
+        )
+        by_day = (
+            base.annotate(day=TruncDate("created_at", tzinfo=tz))
+            .values("day")
+            .annotate(
+                requests=Count("id"),
+                input_tokens=Sum("input_tokens"),
+                output_tokens=Sum("output_tokens"),
+                cost_usd=Sum("cost_usd"),
+                avg_elapsed_ms=Avg("elapsed_ms"),
+            )
+            .order_by("day")
+        )
+        return Response(
+            {
+                "from": d_from.isoformat(),
+                "to": d_to.isoformat(),
+                "totals": {**_row(_agg(base)), "users": base.values("session__user_id").distinct().count(),
+                           "sessions": base.values("session_id").distinct().count()},
+                "byUser": [
+                    {"userId": r["user_id"], "email": r["email"], "name": r["name"] or "", "sessions": r["sessions"], **_row(r)}
+                    for r in by_user
+                ],
+                "byDay": [{"day": r["day"].isoformat(), **_row(r)} for r in by_day],
+            }
+        )