fix: add SNMPv3 credentials to DevicePollerWorker

The DevicePollerWorker was only passing SNMPv2c credentials (community
string) to the SNMP client, causing KeyError when polling SNMPv3 devices.

Changes:
- Updated build_client_opts in device_poller_worker.ex to detect version "3"
- Added same SNMPv3 credential handling as discovery.ex
- Includes: security_name, security_level, auth_protocol, auth_password,
  priv_protocol, priv_password

This completes the SNMPv3 implementation - both discovery and polling now
support v3 authentication alongside the existing v2c support.

lib/towerops/workers/device_poller_worker.ex

@@ -1115,12 +1115,28 @@ defmodule Towerops.Workers.DevicePollerWorker do
   defp build_client_opts(device) do
     snmp_config = Devices.get_snmp_config(device)
 
-    [
+    base_opts = [
       ip: device.ip_address,
-      community: snmp_config.community,
       version: snmp_config.version,
       port: device.snmp_port || 161
     ]
+
+    # Add version-specific credentials
+    if snmp_config.version == "3" do
+      v3_config = Devices.get_snmpv3_config(device)
+
+      base_opts ++
+        [
+          security_name: v3_config.username,
+          security_level: v3_config.security_level,
+          auth_protocol: v3_config.auth_protocol,
+          auth_password: v3_config.auth_password,
+          priv_protocol: v3_config.priv_protocol,
+          priv_password: v3_config.priv_password
+        ]
+    else
+      base_opts ++ [community: snmp_config.community]
+    end
   end
 
   defp get_poll_interval(device) do