Fix cache-control

.github/readme/partials/setup/shared.md

@@ -12,10 +12,12 @@ This is mostly intended for previews, to enjoy all features consider using GitHu
 <summary>💬 Fair use</summary>
 
 To ensure service availability, shared instance has a few limitations:
-  * Images are cached for 1 hour
+  * Images are cached for 15 minutes
     * Rendered metrics **won't be updated** during this time window when queried
     * You can manually update rendering againg your metrics on [metrics.lecoq.io](https://metrics.lecoq.io)
-  * There is a rate limiter enabled (it doesn't affect already cached metrics)
-  * Several plugins may not be available
+  * A rate limiter is enabled to prevent denial of service (it doesn't affect already cached metrics)
+  * Some plugins may not be available
+
+Service is provided free of charge, so please be gentle with it 🙂
 
 </details>

source/app/web/instance.mjs

@@ -69,7 +69,7 @@
       }
     //Cache headers middleware
       middlewares.push((req, res, next) => {
-        res.header("Cache-Control", cached ? `public, max-age=${cached}` : "no-store, no-cache")
+        res.header("Cache-Control", cached ? `public, max-age=${Math.round(cached/1000)}` : "no-store, no-cache")
         next()
       })