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/series/[id].vue

@@ -117,9 +117,11 @@ const handlePurchaseSuccess = () => {
   refreshNuxtData()
 }
 
-useHead(() => ({
-  title: series.value ? `${series.value.title} - Event Series - Ghost Guild` : 'Event Series - Ghost Guild',
-  meta: [{ name: 'description', content: series.value?.description || 'Multi-event series' }],
+useSiteMeta(() => ({
+  title: series.value ? `${series.value.title} · Event Series` : 'Event Series',
+  description:
+    series.value?.description ||
+    (series.value?.title ? `${series.value.title} — a Ghost Guild event series.` : undefined),
 }))
 </script>