feat: relay several servers, registered through the control API

diagrams/pairing.puml

@@ -0,0 +1,29 @@
+@startuml
+skinparam shadowing false
+title Registering a server (pairing)
+
+participant "Admin tool\n(etadm)" as admin
+participant "ET server\n<et-server>:27960" as et
+participant "etrelay control API\n<front-wg>:8090" as control
+participant "Masters" as master
+
+note over admin : picks a key once\n(kept on its side)
+admin -> et : rcon: sets etrelay_proof <fingerprint of the key>
+admin -> control : PUT /server {port: 27960, upstream_port: 27960, key}\nBearer: the relay's token
+control -> control : port offered and free?\n(409: held by another server)
+control -> et : getstatus
+et --> control : statusResponse\n...\\etrelay_proof\\<fingerprint>...
+control -> control : fingerprint == sha256(key)?\n(403: not shown)
+control -> control : bind :27960, keep sha256(key)\nin servers.json
+control --> admin : 200 {port, upstream, since, alive}
+admin -> et : rcon: set etrelay_proof ""
+control -> master : heartbeat EnemyTerritory-1\nfrom :27960
+
+== Later ==
+admin -> control : GET /sessions, /bans...\nBearer: the key (names the server)
+control --> admin : this server's only
+note over admin, control : 401 (the relay lost its state):\nthe admin tool registers again
+admin -> control : DELETE /server
+control -> master : heartbeat ETFlatline-1
+note over control : port freed, bans dropped
+@enduml