fix(docker): COPY favicon assets into image + cache header

Dockerfile selectively COPYs files into /usr/share/nginx/html. Favicon
assets (favicon.svg, favicon-32.png, favicon.ico, apple-touch-icon.png)
were added to the repo in ef31fb3 but never wired into the Dockerfile,
so a rebuilt container served 404 for /favicon.svg and friends — broken
favicon in prod even after `docker compose up -d --build`.

nginx.conf gets a matching long-cache rule for icon/image assets
(30 days, immutable, access_log off) — they rarely change and the file
name is the cache key anyway.

Deploy: on the VPS, `docker compose up -d --build`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nginx.conf

@@ -54,6 +54,13 @@ server {
         add_header Cache-Control "public, max-age=3600, must-revalidate";
     }
 
+    # Long cache for favicon + image assets (rarely change).
+    location ~* \.(?:ico|svg|png|jpg|jpeg|gif|webp)$ {
+        expires 30d;
+        add_header Cache-Control "public, max-age=2592000, immutable";
+        access_log off;
+    }
+
     # Logs to stdout/stderr (default in nginx:alpine).
     access_log /var/log/nginx/access.log;
     error_log /var/log/nginx/error.log warn;