fix: Enrollment-Duplikate, IPv6-Adresse, Agent-Status

- Backend: Enroll-Upsert via MAC-Adresse (kein Duplikat bei Neustart)
  HeartbeatInterval auf 30s reduziert
- Go collector: IPv4 bevorzugen, Loopback/Teredo/ISATAP überspringen,
  Link-Local IPv6 ignorieren
- Go main.go: Enrollment nimmt beste IPv4-Schnittstelle statt Networks[0]

Backend/src/NexusRMM.Api/GrpcServices/AgentGrpcService.cs

@@ -29,6 +29,24 @@ public class AgentGrpcService : AgentService.AgentServiceBase
 
     public override async Task<EnrollResponse> Enroll(EnrollRequest request, ServerCallContext context)
     {
+        // Upsert via MAC-Adresse — verhindert Duplikate bei Agent-Neustart
+        var existing = !string.IsNullOrEmpty(request.MacAddress)
+            ? await _db.Agents.FirstOrDefaultAsync(a => a.MacAddress == request.MacAddress)
+            : null;
+
+        if (existing is not null)
+        {
+            existing.Hostname = request.Hostname;
+            existing.OsVersion = request.OsVersion;
+            existing.IpAddress = request.IpAddress;
+            existing.AgentVersion = request.AgentVersion;
+            existing.Status = AgentStatus.Online;
+            existing.LastSeen = DateTime.UtcNow;
+            await _db.SaveChangesAsync();
+            _logger.LogInformation("Agent re-enrolled: {AgentId} ({Hostname})", existing.Id, existing.Hostname);
+            return new EnrollResponse { AgentId = existing.Id.ToString(), HeartbeatInterval = 30 };
+        }
+
         var agent = new AgentModel
         {
             Id = Guid.NewGuid(),
@@ -47,11 +65,7 @@ public class AgentGrpcService : AgentService.AgentServiceBase
         await _db.SaveChangesAsync();
         _logger.LogInformation("Agent enrolled: {AgentId} ({Hostname})", agent.Id, agent.Hostname);
 
-        return new EnrollResponse
-        {
-            AgentId = agent.Id.ToString(),
-            HeartbeatInterval = 60
-        };
+        return new EnrollResponse { AgentId = agent.Id.ToString(), HeartbeatInterval = 30 };
     }
 
     public override async Task<HeartbeatResponse> Heartbeat(HeartbeatRequest request, ServerCallContext context)