feat(backends+domains): HTTP-Protokoll-Flags — force_http1 + disable_h3

Backend: force_http1 (bool, default false) — zwingt HAProxy auf der
Backend-Verbindung zu HTTP/1.1 statt h2,http/1.1 zu verhandeln.
Nötig für Legacy-Apps die kein h2 sprechen.

Domain: disable_h3 (bool, default false) — unterdrückt Alt-Svc-
Response-Header für diese Domain. Browser erhalten keinen h3/QUIC-
Hinweis und bleiben auf h2/http1.1.

Migration 0027, Model+Service+HAProxy-Template+UI+i18n.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -290,6 +290,8 @@
     "rateLimitHint": "Max requests per second per client IP. HAProxy counts over a 10-second window per stick table (max. 100k IPs). 0 = off.",
     "maxBody": "Max request body",
     "maxBodyHint": "Cap on the Content-Length header. Larger requests get 413. Chunked bodies are not detected (HAProxy doesn't buffer by default). 0 = off.",
+    "disableH3": "Disable HTTP/3 (QUIC)",
+    "disableH3Hint": "Suppresses the Alt-Svc response header for this domain. Browsers won't receive an h3/QUIC upgrade hint and stay on h2/http1.1. Useful when clients report issues with QUIC connections.",
     "headersBtn": "Headers",
     "headersTitle": "Response headers — {{name}}",
     "headersHint": "These headers are set by HAProxy on every response for this domain (http-response set-header). Ordering via position.",
@@ -331,6 +333,8 @@
     "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.",
+    "forceHttp1": "Force HTTP/1.1",
+    "forceHttp1Hint": "Disables HTTP/2 (h2) on the backend connection — HAProxy negotiates HTTP/1.1 only. Required for backends that don't support h2 (e.g. older nginx configs without the h2 module, legacy apps).",
     "servers": "Servers",
     "noServers": "no server",
     "nServers": "{{n}} servers",