Ban lists: where Guard and fail2ban ban addresses

A program bans an address (every port, or a protocol's ports) for a
time; the ban list says what it is kept from: the firewall, some hosts
behind it, or everything. Bans drop ahead of every rule, close the
address's connections, group IPv6 by /64, are kept in the database
across restarts and audited. Never banned: the inside, the firewall's
own addresses, the caller, the list's never. bans grant (bans:write:LIST),
hc banlist/bans/ban/unban, Traffic -> Bans, a fail2ban action in contrib/,
and failed logins and refused tokens logged for Guard and fail2ban.

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

README.md

@@ -53,6 +53,10 @@ table of its own.
   firewall you have, blocking nothing: its hits, log and tracer show what
   it would drop, so you see what switching over would break before you do
   ([docs/shadow.md](docs/shadow.md)).
+* **Ban lists for Guard and fail2ban.** A program on any machine bans an
+  address it caught misbehaving in its logs, and the ban list says what
+  that address is kept from: the firewall, some hosts, or everything
+  ([docs/bans.md](docs/bans.md)).
 * **Easy to watch.** A zone matrix of what is allowed between zones,
   statistics over time (traffic per zone and device, drops, the sources
   blocked most, connections; [docs/stats.md](docs/stats.md)), hits per rule, a live log of what was dropped and
@@ -107,6 +111,7 @@ The web UI is on `http://127.0.0.1:8070/` (see `listen` in
 * [install.md](docs/install.md): packages, building from source, first steps
 * [rules.md](docs/rules.md): zones, address sets, services, rules and the built-ins
 * [nat.md](docs/nat.md): port forwards, hairpin NAT, IPv6 pinholes, source NAT, redirects
+* [bans.md](docs/bans.md): ban lists, where Guard and fail2ban ban addresses
 * [import.md](docs/import.md): moving from an existing nftables ruleset
 * [shadow.md](docs/shadow.md): shadow mode, Housecarl beside your firewall, changing nothing
 * [operations.md](docs/operations.md): probation, the boot and its fallback, hits and the log, monitoring, backups, troubleshooting