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_plan_limits.py

@@ -0,0 +1,58 @@
+"""Integration tests for admin plan limits and account override endpoints."""
+
+import pytest
+from httpx import AsyncClient
+
+
+class TestAdminPlanLimits:
+
+    @pytest.mark.asyncio
+    async def test_list_plan_limits(
+        self, client: AsyncClient, admin_auth_headers: dict
+    ):
+        """List all plan limits."""
+        response = await client.get("/api/v1/admin/plan-limits", headers=admin_auth_headers)
+        assert response.status_code == 200
+        plans = response.json()
+        assert len(plans) >= 3  # free, pro, team seeded in conftest
+        plan_names = [p["plan"] for p in plans]
+        assert "free" in plan_names
+
+    @pytest.mark.asyncio
+    async def test_update_plan_limits(
+        self, client: AsyncClient, admin_auth_headers: dict
+    ):
+        """Update a plan's limits."""
+        response = await client.put(
+            "/api/v1/admin/plan-limits",
+            json={
+                "plan": "free",
+                "max_trees": 5,
+                "max_sessions_per_month": 30,
+                "max_users": 2,
+                "custom_branding": False,
+                "priority_support": False,
+                "export_formats": ["markdown", "text"],
+            },
+            headers=admin_auth_headers,
+        )
+        assert response.status_code == 200
+        data = response.json()
+        assert data["max_trees"] == 5
+
+    @pytest.mark.asyncio
+    async def test_list_account_overrides(
+        self, client: AsyncClient, admin_auth_headers: dict
+    ):
+        """List account overrides."""
+        response = await client.get("/api/v1/admin/account-overrides", headers=admin_auth_headers)
+        assert response.status_code == 200
+        assert isinstance(response.json(), list)
+
+    @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/plan-limits", headers=auth_headers)
+        assert response.status_code == 403