fix: instant CMS uploads + heritage dark/light + ISR caching

Eliminates the need to run "docker compose build" after uploading
images via HQ Command. Heritage page now respects light/dark mode.

CACHE INVALIDATION
- New helper src/lib/revalidate.ts called from /api/assets and
  /api/public-upload after every upload, delete, folder create
- Pages switch from force-dynamic to ISR with revalidate=60
  (regenerated on demand whenever content changes, plus 60s safety)
- Nginx now sends "max-age=300, must-revalidate" instead of "expires 30d"
  on /cases/, /applications/, /news/, /parts/, /footage/, /operations-inbox/
  so browsers revalidate via If-Modified-Since (304s on unchanged files)
- Next.js Image Optimizer aligned with same TTL via minimumCacheTTL=300
  and adds /_next/image location block in Nginx for correct headers

HERITAGE DARK/LIGHT FIX (Bug #8)
- Replaces hardcoded #0A0A0C / #00F0FF / text-white with proper
  light + dark variants throughout markdown renderer (tables, lists,
  headings, blockquotes, paragraphs, images)
- Hero section, navigation pill, and CMS-driven sections now switch
  with the global theme toggle

SECURITY HARDENING
- Server actions bodySizeLimit reduced from 500MB to 50MB
  (large uploads still go through /api/assets which uses Nginx 500MB cap)

DEPLOY NOTES
- Run on VPS:
    git pull
    docker compose up -d --build app
    docker compose exec nginx nginx -s reload
- No DB schema changes in this commit. Existing 2FA users / data untouched.

next.config.ts

@@ -4,14 +4,20 @@ const withNextIntl = createNextIntlPlugin('./src/i18n/request.ts');
 
 const nextConfig = {
   output: "standalone" as const,
-  images: { 
+  images: {
     qualities: [75, 90, 100],
+    // Image Optimizer cache TTL — keeps optimized variants for 5 min,
+    // matching Nginx max-age. Picks up replaced source files quickly.
+    minimumCacheTTL: 300,
+    formats: ['image/avif', 'image/webp'] as ('image/avif' | 'image/webp')[],
   },
   reactStrictMode: true,
   serverExternalPackages: ['nodemailer'],
   experimental: {
     serverActions: {
-      bodySizeLimit: '500mb' as const,
+      // 50MB cap — large enough for hero images and CMS uploads,
+      // small enough to limit DoS surface. Use /api/assets for big files.
+      bodySizeLimit: '50mb' as const,
     },
   },
 };