reduce keepalive timeout

Nearly all of the requests are done automatically in short batches with
long delays in between. Requests aren't made dynamically in response to
user input beyond manually updating or installing apps, so there isn't
much need to optimize for latency.

nginx/nginx.conf

@@ -153,7 +153,7 @@ http {
 
         include root_releases.grapheneos.org.conf;
 
-        keepalive_timeout 3m;
+        keepalive_timeout 15s;
 
         open_file_cache max=2048 inactive=1d;
         open_file_cache_valid 1d;
@@ -286,7 +286,7 @@ http {
 
         include root_apps.grapheneos.org.conf;
 
-        keepalive_timeout 3m;
+        keepalive_timeout 15s;
 
         open_file_cache max=2048 inactive=1d;
         open_file_cache_valid 1d;