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>

internal/license/doc.go

@@ -0,0 +1,5 @@
+// Package license implements the NetCell Licensing client (license.netcell-it.com)
+// with Ed25519 signature verification, KeyDB-lock based leader election (one
+// verify per cluster/day), shared cache in KeyDB, and a 7-day trial fallback.
+// Pattern adopted wholesale from netcell-webpanel/docs/licensing-integration.md.
+package license