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/tests/test_stats.py

@@ -0,0 +1,60 @@
+"""GET /api/v1/admin/stats — 관리자만, 토큰·비용·시간 집계."""
+
+from datetime import timedelta
+
+import pytest
+from django.utils import timezone
+from rest_framework.test import APIClient
+
+from apps.chat.models import ChatMessage, ChatSession
+
+
+def _msg(session, role="assistant", days_ago=0, **kw):
+    m = ChatMessage.objects.create(session=session, role=role, content="x", **kw)
+    if days_ago:
+        ChatMessage.objects.filter(id=m.id).update(created_at=timezone.now() - timedelta(days=days_ago))
+    return m
+
+
+@pytest.fixture
+def admin_api(db, django_user_model):
+    admin = django_user_model.objects.create_superuser(username="a@x.com", email="a@x.com", password="x")
+    c = APIClient()
+    c.force_authenticate(admin)
+    return c, admin
+
+
+def test_non_admin_forbidden(auth_api):
+    res = auth_api.get("/api/v1/admin/stats")
+    assert res.status_code == 403 and res.json()["code"] == "FORBIDDEN"
+
+
+def test_totals_by_user_by_day(admin_api, user):
+    api, admin = admin_api
+    s1 = ChatSession.objects.create(id="s1", user=user)
+    s2 = ChatSession.objects.create(id="s2", user=admin)
+    _msg(s1, role="user")  # user 행은 집계 제외
+    _msg(s1, input_tokens=100, output_tokens=50, cost_usd=0.01, elapsed_ms=1000)
+    _msg(s1, input_tokens=200, output_tokens=100, cost_usd=0.02, elapsed_ms=3000, days_ago=1)
+    _msg(s2, input_tokens=10, output_tokens=5, cost_usd=0.001, elapsed_ms=500)
+    _msg(s2, input_tokens=999, output_tokens=999, cost_usd=9.0, elapsed_ms=9, days_ago=40)  # 기간 밖
+
+    data = api.get("/api/v1/admin/stats").json()["data"]
+    t = data["totals"]
+    assert t["requests"] == 3 and t["inputTokens"] == 310 and t["outputTokens"] == 155 and t["totalTokens"] == 465
+    assert t["costUsd"] == pytest.approx(0.031) and t["avgElapsedMs"] == 1500
+    assert t["users"] == 2 and t["sessions"] == 2
+
+    by_user = {r["email"]: r for r in data["byUser"]}
+    assert by_user[user.email]["requests"] == 2 and by_user[user.email]["totalTokens"] == 450
+    assert by_user[user.email]["sessions"] == 1 and by_user[user.email]["avgElapsedMs"] == 2000
+    assert data["byUser"][0]["email"] == user.email  # 비용 큰 순
+    assert len(data["byDay"]) == 2 and sum(r["requests"] for r in data["byDay"]) == 3
+
+
+def test_range_validation(admin_api):
+    api, _ = admin_api
+    assert api.get("/api/v1/admin/stats?from=2026-13-01").status_code == 400
+    assert api.get("/api/v1/admin/stats?from=2026-09-10&to=2026-09-01").status_code == 400
+    res = api.get("/api/v1/admin/stats?from=2026-09-01&to=2026-09-02")
+    assert res.status_code == 200 and res.json()["data"]["from"] == "2026-09-01"