My first thought is to add these lines towards the bottom of the file:
Code:
# BLOCK IT ALL ON INTERNAL NIC
block on $int_if all
# REDIRECT http, https LAN TRAFFIC TO PROXY SERVER
pass in on $int_if proto tcp from $lan to any port { http https } \
rdr-to $proxy
# LET http, https OUT FROM PROXY SERVER
pass out on $proxy proto tcp to any port { http https }