Only track bannable (globally-routable) source IPs

The ban logic is per source IP, so it only works where the daemon can see
the real client. Behind Docker's default bridge networking every client is
SNAT'd to the bridge gateway (a 172.16/12 address), so a single IP would
stand in for the whole internet -- counting offenses against it would block
everyone at once.

Add is_bannable_address(): only globally-routable unicast addresses are
tracked. Loopback, RFC1918 private, CGNAT (100.64/10), link-local, IPv6
unique-local, and multicast all return false. main.cpp decides trackability
from the accepted endpoint and skips both the block check and offense
recording for non-global sources. Net effect: banning works where the real
IP is visible (FreeBSD jail via pf rdr; Docker with host networking) and is
inert -- not catastrophic -- where it is not (Docker bridge).

Document the Docker client-IP caveat: docker-compose.yml now defaults to
host networking, with the rationale and alternatives in DOCKER.md.

ban.hpp

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <boost/asio/ip/address.hpp>
 #include <chrono>
 #include <cstddef>
 #include <deque>
@@ -58,3 +59,17 @@ private:
   // monotonic, so appends are always newest-last).
   std::unordered_map<std::string, std::deque<clock::time_point>> offenders_;
 };
+
+// Whether a client address is meaningful to track and ban. Only globally
+// routable unicast addresses qualify. Loopback, RFC1918 private, CGNAT
+// (100.64/10), link-local, IPv6 unique-local, and multicast addresses all
+// return false.
+//
+// This matters because the daemon can only ban what it can see: behind Docker's
+// default bridge networking every external client is SNAT'd to the bridge
+// gateway (a 172.16/12 address), so banning per source IP would collapse all
+// clients into one and block everyone. Skipping non-global addresses makes
+// banning correct where the real client IP is visible (e.g. the FreeBSD jail,
+// where pf rdr preserves it) and inert where it is not (Docker bridge), with no
+// deployment-specific configuration.
+bool is_bannable_address(const boost::asio::ip::address &addr);