feat: initial etrelay UDP relay with master heartbeat

diagrams/registration.puml

@@ -0,0 +1,28 @@
+@startuml
+skinparam shadowing false
+title Master registration (every 5 min)
+
+participant "Master\netmaster.net:27950" as master
+participant "etrelay\nfront :27960" as relay
+participant "ET server\n(via wg + nginx)" as et
+
+== Liveness probe (dedicated ephemeral socket) ==
+relay -> et : getinfo etrelay
+et --> relay : infoResponse
+note right of relay : up: send heartbeat\nno answer x3: skip (or flatline once)
+
+== Heartbeat (public socket) ==
+relay -> master : heartbeat EnemyTerritory-1\nsource = front:27960
+
+== Master checks: handled like any client ==
+master -> relay : getChallenge\nto front:27960
+relay -> relay : new session for master:27950
+relay -> et : getChallenge
+et --> relay : challengeResponse
+relay --> master : challengeResponse\nsource = front:27960
+master -> relay : getInfo
+relay -> et : getInfo
+et --> relay : infoResponse (hostname, map, players)
+relay --> master : infoResponse\nsource = front:27960
+note left of master : lists <front-ip>:27960
+@enduml