feat(09-01): implement GET /api/admin/overview endpoint — ADMIN-11

- New backend/api/admin/overview.py: aggregate stats (user_count,
  total_storage_bytes, doc_status) + last 10 audit entries in one payload
- Sub-router carries NO prefix (parent __init__.py carries /api/admin)
- Reuses _build_filtered_query_with_handles + _audit_to_dict_with_handles
  from api.audit — no new serializer or query logic
- Registered overview_router on admin parent router in __init__.py
- All 8 ADMIN-11 tests pass; existing admin and audit suites stay green

backend/api/admin/overview.py

@@ -0,0 +1,52 @@
+from __future__ import annotations
+
+from fastapi import APIRouter, Depends
+from sqlalchemy import func, select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from db.models import AuditLog, Document, Quota, User
+from deps.auth import get_current_admin
+from deps.db import get_db
+from api.audit import _build_filtered_query_with_handles
+from api.audit import _audit_to_dict_with_handles
+
+router = APIRouter()  # NO prefix — parent __init__.py carries /api/admin (D-02)
+
+
+@router.get("/overview")
+async def get_overview(
+    session: AsyncSession = Depends(get_db),
+    _admin: User = Depends(get_current_admin),
+) -> dict:
+    user_count = await session.scalar(
+        select(func.count(User.id)).where(User.role == "user")
+    ) or 0
+
+    total_storage_bytes = await session.scalar(
+        select(func.sum(Quota.used_bytes))
+    ) or 0
+
+    status_rows = (
+        await session.execute(
+            select(Document.status, func.count(Document.id)).group_by(Document.status)
+        )
+    ).all()
+    doc_status = {row[0]: row[1] for row in status_rows}
+
+    audit_q = (
+        _build_filtered_query_with_handles(None, None, None, None)
+        .order_by(AuditLog.created_at.desc())
+        .limit(10)
+    )
+    audit_rows = (await session.execute(audit_q)).all()
+    recent_audit = [
+        _audit_to_dict_with_handles(row[0], row[1], row[2], row[3])
+        for row in audit_rows
+    ]
+
+    return {
+        "user_count": user_count,
+        "total_storage_bytes": total_storage_bytes,
+        "doc_status": doc_status,
+        "recent_audit": recent_audit,
+    }