feat(license): Lizenz-System mit Ed25519-Verify gegen license.netcell-it.com

Portiert mail-gateway/internal/license (Verify, Cache, Trial, Signature)
+ DB-Mirror (internal/services/license) + REST-Handler (status/verify/key/clear)
+ UI-Page /license (Activate, Status, Limits, Features, Re-verify)
+ <LicenseBanner /> neben UpdateBanner (trial-expiring, expired, verify-failed)
+ Scheduler: täglich Re-verify (24h-Tick)

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

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

@@ -18,6 +18,7 @@
     "ntp": "Time (NTP)",
     "firewall": "Firewall",
     "cluster": "Cluster",
+    "license": "License",
     "settings": "Settings",
     "section": {
       "overview": "Overview",
@@ -551,5 +552,42 @@
     "download": "Download",
     "copy": "Copy",
     "copied": "Copied"
+  },
+  "license": {
+    "title": "License",
+    "status": "Status",
+    "product": "Product",
+    "key": "License key",
+    "noKey": "No key configured",
+    "validUntil": "Valid until",
+    "expired": "Expired",
+    "daysLeft": "{{days}} days left",
+    "lastVerifiedAt": "Last verified",
+    "verifiedBy": "Verified by",
+    "limits": "Limits",
+    "unlimited": "Unlimited",
+    "features": "Features",
+    "reverify": "Re-verify",
+    "reverified": "License re-verified successfully",
+    "enterKey": "Enter key",
+    "replaceKey": "Replace key",
+    "enterKeyHint": "Paste your license key from the self-service portal at license.netcell-it.com.",
+    "activate": "Activate",
+    "saved": "License saved and verified",
+    "savedButVerifyFailed": "Key saved but server-verify failed",
+    "clearKey": "Remove key",
+    "cleared": "License removed — system falls back to trial",
+    "confirmClear": "Really remove the license key?",
+    "confirmClearHint": "The system will fall back to trial-mode once the key is deleted.",
+    "lastVerifyFailed": "Last server verify failed",
+    "trialExpiring": "Trial expires in {{days}} day(s)",
+    "trialExpiringHint": "Activate a license before the trial period ends."
+  },
+  "licenseBanner": {
+    "expired": "License expired or invalid.",
+    "trialExpiring": "Trial expires in {{days}} day(s).",
+    "verifyFailed": "License verification failed",
+    "cta": "Activate now →",
+    "openPage": "Open license page →"
   }
 }