fix(forgejo): lower cache.last_commit TTL to limit size of the cache

We really don't want to cache them for a year, which is the default.

Yes, computing them may be expensive, but not worth a multi-gigabyte
redis database that takes minutes to load into RAM on service (re)start.

services/forgejo/default.nix

@@ -73,6 +73,10 @@ in
           ITEM_TTL = "72h"; # increased from default 16h
         };
 
+        "cache.last_commit" = {
+          ITEM_TTL = "24h"; # from default 8760h (1 year)
+        };
+
         ui = {
           SHOW_USER_EMAIL = false;
         };