feat: add admin dashboard, audit logs, token expiry check and test suite

Frontend Features:
- Add ProtectedRoute component with token expiry validation
- Create AdminDashboardPage with system statistics and user management
- Create AuditLogsPage with filtering and pagination
- Add admin-only navigation (Shield icon) for ymirliu@panjit.com.tw
- Add admin API methods to apiV2 service
- Add admin type definitions (SystemStats, AuditLog, etc.)

Token Management:
- Auto-redirect to login on token expiry
- Check authentication on route change
- Show loading state during auth check
- Admin privilege verification

Backend Testing:
- Add pytest configuration (pytest.ini)
- Create test fixtures (conftest.py)
- Add unit tests for auth, tasks, and admin endpoints
- Add integration tests for complete workflows
- Test user isolation and admin access control

Documentation:
- Add TESTING.md with comprehensive testing guide
- Include test running instructions
- Document fixtures and best practices

Routes:
- /admin - Admin dashboard (admin only)
- /admin/audit-logs - Audit logs viewer (admin only)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

backend/tests/test_admin.py

@@ -0,0 +1,56 @@
+"""
+Unit tests for admin endpoints
+"""
+
+import pytest
+
+
+class TestAdmin:
+    """Test admin endpoints"""
+
+    def test_get_system_stats(self, client, admin_token):
+        """Test get system statistics"""
+        response = client.get(
+            '/api/v2/admin/stats',
+            headers={'Authorization': f'Bearer {admin_token}'}
+        )
+
+        assert response.status_code == 200
+        data = response.json()
+        assert 'total_users' in data
+        assert 'total_tasks' in data
+        assert 'task_stats' in data
+
+    def test_get_system_stats_non_admin(self, client, auth_token):
+        """Test that non-admin cannot access admin endpoints"""
+        response = client.get(
+            '/api/v2/admin/stats',
+            headers={'Authorization': f'Bearer {auth_token}'}
+        )
+
+        assert response.status_code == 403
+
+    def test_list_users(self, client, admin_token):
+        """Test list all users"""
+        response = client.get(
+            '/api/v2/admin/users',
+            headers={'Authorization': f'Bearer {admin_token}'}
+        )
+
+        assert response.status_code == 200
+        data = response.json()
+        assert 'users' in data
+        assert 'total' in data
+
+    def test_get_audit_logs(self, client, admin_token):
+        """Test get audit logs"""
+        response = client.get(
+            '/api/v2/admin/audit-logs',
+            headers={'Authorization': f'Bearer {admin_token}'}
+        )
+
+        assert response.status_code == 200
+        data = response.json()
+        assert 'logs' in data
+        assert 'total' in data
+        assert 'page' in data