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.

server/utils/og-cache.js

@@ -0,0 +1,37 @@
+import { mkdir, readFile, writeFile } from 'node:fs/promises'
+import { dirname, resolve } from 'node:path'
+import { fileURLToPath } from 'node:url'
+
+const __dirname = dirname(fileURLToPath(import.meta.url))
+const CACHE_DIR = resolve(__dirname, '../../.cache/og')
+
+let ensured = false
+
+async function ensureCacheDir() {
+  if (ensured) return
+  await mkdir(CACHE_DIR, { recursive: true })
+  ensured = true
+}
+
+// Returns cached PNG buffer or null. Key must be safe for a filename.
+export async function getCachedOG(key) {
+  await ensureCacheDir()
+  try {
+    return await readFile(resolve(CACHE_DIR, `${key}.png`))
+  } catch {
+    return null
+  }
+}
+
+export async function setCachedOG(key, buffer) {
+  await ensureCacheDir()
+  await writeFile(resolve(CACHE_DIR, `${key}.png`), buffer)
+}
+
+// Build a safe, deterministic cache key from event slug + updatedAt timestamp.
+// Slug is already URL-safe; updatedAt busts the cache on any edit.
+export function eventCacheKey(event) {
+  const slug = String(event.slug || event._id).replace(/[^a-zA-Z0-9_-]/g, '_')
+  const stamp = event.updatedAt ? new Date(event.updatedAt).getTime() : 0
+  return `event-${slug}-${stamp}`
+}