Build Housecarl 0.1.0: the nftables firewall manager

Zones, address sets (IPv4 and IPv6 mixed, dynamic sets with timeouts) and
services, ordered rules in the early, input, forward and output chains,
port forwards with hairpin NAT and IPv6 pinholes, source NAT, redirects,
and switchable built-ins (loopback, conntrack, ICMPv6, antispoofing, MSS
clamping, DHCPv6 client, flowtable). Every change is validated, checked
with nft -c and applied in one transaction to Housecarl's own table,
inet housecarl; changes made over the network roll back unless
confirmed, and the confirmed ruleset is loaded at boot by
housecarl-ruleset.service. An empty configuration applies nothing.

Also: hit counters and the drop log (nflog), 'hc import' of an existing
nftables ruleset and 'hc takeover', the hc CLI, the REST API and the web
UI, users, scoped tokens (sets:write:NAME), the audit log, the web UI's
certificate through Jarl, Debian packages, docs, unit tests with real
nft 1.1.3, and an e2e test with real traffic (71 checks).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

deploy/debian/prerm

@@ -0,0 +1,16 @@
+#!/bin/sh
+# Housecarl: runs before the package's files are removed (also on upgrades).
+set -e
+
+# On removal (not on upgrade) stop housecarld and the backup timer and remove
+# the boot-time links (postrm deals with the timer's). The ruleset in force
+# stays until the next boot; then the machine has no Housecarl table.
+if [ "$1" = remove ] && [ -d /run/systemd/system ]; then
+	systemctl stop housecarl.service housecarl-backup.timer housecarl-backup.service 2>/dev/null || true
+	systemctl disable housecarl.service housecarl-ruleset.service 2>/dev/null || true
+	if ! command -v deb-systemd-helper >/dev/null 2>&1; then
+		systemctl disable housecarl-backup.timer 2>/dev/null || true
+	fi
+fi
+
+exit 0