feat: implement full admin panel with dashboard, user management, and platform settings

Adds complete super_admin panel with 9 pages and account owner categories page.
Backend includes 5 new DB tables, ~25 API endpoints, settings manager with
in-memory cache, and 29 integration tests. Frontend includes reusable admin
components (DataTable, Pagination, ActionMenu, etc.) with code-split lazy loading.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/tests/test_admin_settings.py

@@ -0,0 +1,43 @@
+"""Integration tests for admin settings endpoints."""
+
+import pytest
+from httpx import AsyncClient
+
+
+class TestAdminSettings:
+
+    @pytest.mark.asyncio
+    async def test_list_settings(
+        self, client: AsyncClient, admin_auth_headers: dict
+    ):
+        """List platform settings (may be empty if not seeded via migration)."""
+        response = await client.get("/api/v1/admin/settings", headers=admin_auth_headers)
+        assert response.status_code == 200
+        data = response.json()
+        assert "settings" in data
+        assert isinstance(data["settings"], dict)
+
+    @pytest.mark.asyncio
+    async def test_update_settings(
+        self, client: AsyncClient, admin_auth_headers: dict
+    ):
+        """Update maintenance_mode setting."""
+        response = await client.put(
+            "/api/v1/admin/settings",
+            json={"settings": {"maintenance_mode": "true"}},
+            headers=admin_auth_headers,
+        )
+        assert response.status_code == 200
+
+        # Verify change
+        get_resp = await client.get("/api/v1/admin/settings", headers=admin_auth_headers)
+        settings = get_resp.json()["settings"]
+        assert settings["maintenance_mode"] is True or settings["maintenance_mode"] == "true"
+
+    @pytest.mark.asyncio
+    async def test_non_admin_cannot_access(
+        self, client: AsyncClient, auth_headers: dict
+    ):
+        """Non-admin gets 403."""
+        response = await client.get("/api/v1/admin/settings", headers=auth_headers)
+        assert response.status_code == 403