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/board.vue

@@ -192,14 +192,10 @@ const loadTags = async () => {
   cooperativeTags.value = (data.tags || []).filter((t) => t.pool === 'cooperative')
 }
 
-useHead({
-  title: 'Board - Ghost Guild',
-  meta: [
-    {
-      name: 'description',
-      content: 'Share what you are seeking and offering with the Ghost Guild community.',
-    },
-  ],
+useSiteMeta({
+  title: 'Bulletin Board',
+  description:
+    'The Ghost Guild bulletin board. Members post offers and requests around shared interests and cooperative topics.',
 })
 
 onMounted(async () => {