feat(update): Testing/Stable-Update-Kanäle wie enconf (Suite=trixie, Komponente=Kanal)

Installer (EDGEGUARD_CHANNEL), Kanal-Lesen/-Schreiben ohne DB-State
(sources.list ist Quelle der Wahrheit), Cluster-Endpoints für Kanalwechsel
mit mTLS-Peer-Propagation + Drift-Erkennung, --allow-downgrades für
testing→stable-Downgrades über den bestehenden sicheren Rolling-Update-
Flow, Settings-UI mit Bestätigung, neues scripts/release.sh (Testing-Push
datumsbasiert YYYY.MM.DD.NN, Stable-Promotion mit Verify-Gate + Git-Tag),
publish.sh/cleanup-old.sh kanalfähig mit Stable-Tag-Schutz.

Migriert Bestandsnodes automatisch von der alten "main"-Komponente auf
"stable" (postinst, idempotent) — ohne das würden vor diesem Release
installierte Nodes stillschweigend keine Updates mehr sehen, sobald
main nicht mehr bespielt wird.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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

@@ -925,6 +925,17 @@
     "autoUpdateHint": "Whitelist covers edgeguard, edgeguard-api, edgeguard-ui only. Other packages stay under manual control. Requires unattended-upgrades (Trixie distro default). Conf file: /etc/apt/apt.conf.d/52edgeguard-auto-updates.",
     "autoUpdateToggled": "Auto-update setting saved.",
     "autoUpdateFailed": "Auto-update toggle failed",
+    "updateChannelCardTitle": "Update channel",
+    "updateChannelStable": "Stable",
+    "updateChannelTesting": "Testing",
+    "updateChannelApply": "Apply",
+    "updateChannelConfirmTitle": "Switch update channel?",
+    "updateChannelSwitchTestingWarn": "Testing may contain unstable interim builds. Both nodes will be switched.",
+    "updateChannelSwitchStableWarn": "Switching back to stable may trigger a downgrade on both nodes (testing versions are dated newer).",
+    "updateChannelSaved": "Update channel saved (both nodes).",
+    "updateChannelFailed": "Update channel switch failed",
+    "updateChannelDrift": "Channel drift detected against the peer node (peer: {{peer}}) — will sync on the next switch.",
+    "updateChannelHint": "Testing pulls date-stamped interim builds from the testing repo, stable pulls curated releases. The channel applies to both HA nodes in sync.",
     "ipv6CardTitle": "IPv6",
     "ipv6On": "Enabled — HAProxy binds on [::]:80, [::]:443 and [::]:3443 in addition to IPv4.",
     "ipv6Off": "Disabled — HAProxy listens on IPv4 only.",