Discovery with SNMPv3 was completing in 52 seconds, dangerously close
to the 60s timeout. Increasing to 120s provides comfortable headroom
for complete MikroTik OID tree traversal with encryption overhead.
SNMPv3 has significant encryption and authentication overhead compared to
v2c. MikroTik enterprise tree walks (1.3.6.1.4.1.14988) can take 16+
seconds with SNMPv3, causing timeouts with the previous 30s limit.
Increased timeout from 30s to 60s in both client.rs and device_poller.rs
to accommodate SNMPv3's additional latency.
