feat: add proxy_cache in /dev/shm for Plausible script

- Cache size: 1m keysZone, 100m max
- TTL: 5 minutes
- Cache hit/miss tracked via X-Cache header

Author: mroderick

config/nginx.conf.erb

@@ -12,6 +12,9 @@ http {
   # DNS resolver for proxy_pass
   resolver 9.9.9.9 valid=30s;
 
+  # Proxy cache in /dev/shm (tmpfs, persists across dyno restarts)
+  proxy_cache_path /dev/shm/nginx_cache levels=1:2 keys_zone=plausible_cache:1m max_size=100m inactive=5m use_temp_path=off;
+
   # Security headers
   add_header X-Frame-Options "SAMEORIGIN" always;
   add_header X-Content-Type-Options "nosniff" always;
@@ -39,11 +42,17 @@ http {
     set $plausible_script_url https://plausible.io/js/pa-PFruVsE_br97UUCRXE_6f.js;
     set $plausible_event_url https://plausible.io/api/event;
 
-    # Plausible: Proxy script.js
+    # Plausible: Proxy script.js (cached)
     location = /js/script.js {
+      proxy_cache plausible_cache;
+      proxy_cache_valid 200 5m;
+      proxy_cache_key "$host$uri";
       proxy_pass $plausible_script_url;
       proxy_set_header Host plausible.io;
       proxy_buffering on;
+
+      # Cache response headers
+      add_header X-Cache $upstream_cache_status;
     }
 
     # Plausible: Proxy event API