feat(seo): site meta composable + Open Graph image generation

Adds `useSiteMeta()` composable that wraps useSeoMeta with site defaults
(title template, canonical URL, og/twitter image, og:site_name) and
absolute-URL handling via runtimeConfig.public.appUrl.

Adds /og/events/[slug].png route that renders per-event OG images via
satori + @resvg/resvg-js, cached on disk by slug + updatedAt. Bundles
Brygada 1918 + Commit Mono fonts as server assets, ships a fallback
default.png, and patches @shuding/opentype.js via patch-package.

Converts ~25 pages from useHead to useSiteMeta and adds noindex on
private/auth/admin pages.

app/pages/auth/logout-confirm.vue

@@ -1,6 +1,6 @@
 <script setup lang="ts">
 definePageMeta({ layout: false });
-useHead({ title: "Sign Out — Ghost Guild" });
+useSiteMeta({ title: "Sign Out", noindex: true });
 
 // The xsrf token comes from a short-lived httpOnly cookie set by
 // oidc-provider's logoutSource callback (see server/utils/oidc-provider.ts).