fix: security and stability fixes from code review

- Scope WebSocket auth bypass to only WS endpoints by restructuring
  routes into separate chi Groups. Non-WS routes no longer passthrough
  unauthenticated requests with spoofed Upgrade headers. Added
  optionalAPIKeyOrJWT middleware for WS routes (injects auth context
  from API key/JWT if present, passes through otherwise) and
  markAdminWS middleware for admin WS routes.

- Fix nil pointer dereference in envd Handler.Wait() — p.tty.Close()
  was called unconditionally but p.tty is nil for non-PTY processes,
  crashing every non-PTY process exit.

- Fix goroutine leak in sandbox Pause — stopSampler was never called,
  leaking one sampler goroutine per successful pause operation.

- Decouple PTY WebSocket reads from RPC dispatch using a buffered
  channel to prevent backpressure-induced connection drops under fast
  typing. Includes input coalescing to reduce RPC call volume.

internal/sandbox/manager.go

@@ -363,6 +363,11 @@ func (m *Manager) Pause(ctx context.Context, sandboxID string) error {
 		return fmt.Errorf("sandbox %s is not running (status: %s)", sandboxID, sb.Status)
 	}
 
+	// Stop the metrics sampler goroutine before tearing down any resources
+	// it reads (dm device, Firecracker PID). Without this, the sampler
+	// leaks on every successful pause.
+	m.stopSampler(sb)
+
 	// Step 0: Drain in-flight proxy connections before freezing vCPUs.
 	// This prevents Go runtime corruption inside the guest caused by stale
 	// TCP state from connections that were alive when the VM was snapshotted.