fix: only send SNMP jobs to agent for SNMP-enabled devices (#180)

- Check device.snmp_enabled before creating discovery/polling jobs
- Also check snmp_enabled for MikroTik jobs
- Prevents agent from attempting SNMP operations on ping-only devices
- Fixes "Agent collected 0 OIDs" errors for devices without SNMP

Reviewed-on: graham/towerops-web#180

lib/towerops_web/channels/agent_channel.ex

@@ -969,18 +969,20 @@ defmodule ToweropsWeb.AgentChannel do
   end
 
   defp build_jobs_for_device(device) do
-    # Build SNMP job (discovery or polling)
+    # Build SNMP job (discovery or polling) only if SNMP is enabled
     snmp_job =
-      if needs_discovery?(device) do
-        build_discovery_job(device)
-      else
-        build_polling_job(device)
+      if device.snmp_enabled do
+        if needs_discovery?(device) do
+          build_discovery_job(device)
+        else
+          build_polling_job(device)
+        end
       end
 
     # Build MikroTik job only during discovery (not during regular polling)
     # MikroTik commands are discovery-type operations that should run once per 24h
     mikrotik_job =
-      if needs_discovery?(device) and mikrotik_device?(device) do
+      if device.snmp_enabled and needs_discovery?(device) and mikrotik_device?(device) do
         build_mikrotik_job(device)
       end