fix(users/Profpatsch/openlab-tools): add cache headers

This is a dumb experiment to see how hard it is to respect cache
headers; turns out, medium hard but doable.

Sets the correct expiry time according to the cache, plus respects
`If-Modified-Since` which is a tiny bit harder.

Change-Id: I9e6166af0fa254df2beb0f3919187b91a407487b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9810
Autosubmit: Profpatsch <mail@profpatsch.de>
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>

users/Profpatsch/cabal.project

@@ -9,6 +9,7 @@ packages:
   ./cas-serve/cas-serve.cabal
   ./jbovlaste-sqlite/jbovlaste-sqlite.cabal
   ./whatcd-resolver/whatcd-resolver.cabal
+  ./openlab-tools/openlab-tools.cabal
   ./ircmail/ircmail.cabal
   ./httzip/httzip.cabal
   ./declib/declib.cabal