Fix security vulnerabilities across backend, frontend, and infra

- Fix auth bypass in UpdatePost/DeletePost (missing return after auth check)
- Remove Spotify access token from callback response
- Replace internal error messages with generic responses in all handlers
- Harden GraphQL: complexity limit, disable playground/introspection in prod
- Add security headers (X-Frame-Options, HSTS, etc.) to nginx
- Disable Hasura console/dev mode in production
- Add DOMPurify sanitization to Markdown component
- Fix cookie removal to use correct domain/path from auth config
- Fix nil dereference in rowing handler when Claude API errors
- Fix wildcard CORS on stamp endpoint
- Pin nginx and certbot Docker image versions

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

nginx/nginx.conf.template

@@ -77,6 +77,11 @@ http {
 		ssl_certificate /etc/letsencrypt/live/$DOMAIN/fullchain.pem;
 		ssl_certificate_key /etc/letsencrypt/live/$DOMAIN/privkey.pem;
 
+        add_header X-Frame-Options "SAMEORIGIN" always;
+        add_header X-Content-Type-Options "nosniff" always;
+        add_header Referrer-Policy "strict-origin-when-cross-origin" always;
+        add_header Strict-Transport-Security "max-age=63072000; includeSubDomains" always;
+
         # Vite hashed assets - immutable, cache 1 year
         location /assets/ {
             expires 1y;
@@ -117,7 +122,7 @@ http {
         }
 
         location = /img/stamps/mine.gif {
-            add_header Access-Control-Allow-Origin *;
+            add_header Access-Control-Allow-Origin "https://www.$DOMAIN";
             expires 7d;
             add_header Cache-Control "public";
         }