fix(web): Set metadataBase to fix og:image URL behind reverse proxy

Without metadataBase, Next.js falls back to the raw HTTP request host
(localhost:8080) when constructing the absolute og:image URL from
opengraph-image.tsx. Setting metadataBase to AUTH_URL ensures the
public-facing URL is used instead.

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

Author: brendan-kellam

packages/web/src/app/layout.tsx

@@ -11,6 +11,7 @@ import { PlanProvider } from "@/features/entitlements/planProvider";
 import { getEntitlements } from "@sourcebot/shared";
 
 export const metadata: Metadata = {
+  metadataBase: new URL(env.AUTH_URL),
   // Using the title.template will allow child pages to set the title
   // while keeping a consistent suffix.
   title: {