feat: relay several servers, registered through the control API

diagrams/tasks.puml

@@ -2,14 +2,18 @@
 skinparam shadowing false
 title Tasks and data flow
 
-rectangle "relay loop (main task)" as loop {
+rectangle "relay loop (one per server)" as loop {
   card "public socket\nrecv_from" as recv
   card "filter\n(commands, query rate)" as filter
 }
 
-rectangle "shared state (Mutex)" as shared {
+rectangle "server state (Mutex, one per server)" as shared {
   card "sessions:\nHashMap<client, Session>" as map
-  card "bans\n(bans.json)" as bans
+  card "bans\n(bans-<port>.json)" as bans
+}
+
+rectangle "registry (Mutex)" as registry {
+  card "servers by public port,\ntheir key's hash\n(servers.json)" as servers
 }
 
 rectangle "session task (one per client)" as st {
@@ -17,12 +21,13 @@ rectangle "session task (one per client)" as st {
   card "upstream socket\n(connected to ET)" as up
 }
 
-rectangle "heartbeat task" as hb {
+rectangle "heartbeat task (one per server)" as hb {
   card "interval 5 min" as tick
   card "probe socket" as probe
 }
 
 rectangle "control API task (axum)" as ctl {
+  card "PUT/GET/DELETE /server" as pairing
   card "GET /sessions\nGET/PUT/DELETE /bans" as routes
 }
 
@@ -36,4 +41,7 @@ tick --> probe : getinfo
 tick --> recv : heartbeat via public socket
 routes --> map : list ports,\ndrop a banned IP's sessions
 routes --> bans : set / lift
+routes --> servers : key -> server
+pairing --> servers : register / leave
+servers --> loop : start / stop\n(with its heartbeat)
 @enduml