feat(backends): WebSocket-Toggle pro Backend

Migration 0017 fügt backends.websocket BOOL. Wenn aktiv emittiert der
HAProxy-Renderer `timeout tunnel 1h` IM Backend-Block; defaults-Section
hat den Global-Timeout dafür verloren. Backends ohne WS-Workload bleiben
bei strikten HTTP-Timeouts (Connection-Hygiene). Migrations-Heuristik
schaltet vm-pool/proxmox/console/vnc-Namen auto auf true damit Proxmox-
Konsole nach Deploy weiterhin durchläuft.

UI: Switch im Backend-Modal + WS-Tag in der Übersichtstabelle.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

management-ui/src/i18n/locales/en/common.json

@@ -217,6 +217,8 @@
     "selectDomains": "Select domains",
     "lbAlgo": "Load balancing",
     "lbAlgoHint": "roundrobin = evenly, leastconn = pick the server with fewest active connections, source = sticky per client-IP hash (for stateful apps without shared session).",
+    "websocket": "WebSocket support",
+    "websocketHint": "On: allow long-lived WebSocket / long-poll connections (Proxmox console, SSH-over-WS, AsyncAPI) — tunnel idle 1h instead of 60s. Off: strict HTTP timeouts.",
     "servers": "Servers",
     "noServers": "no server",
     "nServers": "{{n}} servers",