fix: use svix library for Polar webhook signature verification

package.json

@@ -15,6 +15,7 @@
     "@dnd-kit/sortable": "^10.0.0",
     "@dnd-kit/utilities": "^3.2.2",
     "@hookform/resolvers": "^5.2.2",
+    "@polar-sh/sdk": "^0.47.1",
     "@radix-ui/react-accordion": "^1.2.12",
     "@radix-ui/react-avatar": "^1.1.11",
     "@radix-ui/react-checkbox": "^1.3.3",
@@ -55,6 +56,7 @@
     "react-resizable-panels": "^3.0.4",
     "recharts": "3.6.0",
     "sonner": "^2.0.7",
+    "svix": "^1.92.2",
     "tailwind-merge": "^3.4.0",
     "vaul": "^1.1.2",
     "zod": "^4.3.2",