When the WebSocket connection drops, session cleanup waits for all
worker pools to drain. SNMP/MikroTik operations against unresponsive
devices can block for 30+ seconds per worker, preventing the agent
from reconnecting during that time.
Add stopWithTimeout(5s) so pool shutdown is bounded. If workers don't
finish within the timeout, they're abandoned and reconnection proceeds
immediately.
Fix 1: Limit inbound job payload size to 4MB to prevent OOM from malicious server
Fix 2: Worker pool submit accepts context and returns bool, prevents blocking event loop
Fix 3: Panic recovery in worker pool goroutines prevents permanent worker death
Fix 4: WebSocket handshake timeout (30s) prevents indefinite blocking on slow server
Fix 5: Writer goroutine propagates errors to main loop via writeErrCh
Replace unbounded goroutine spawning with fixed-size worker pools:
100 workers for SNMP, 20 for MikroTik, 50 for ping. Prevents file
descriptor exhaustion and device overload when handling thousands
of concurrent jobs. Pool provides backpressure when saturated.
