Implement network throughput monitoring instead of cumulative bytes

- Add iftop as runtime dependency in package.nix and flake.nix
- Modify SystemMetrics to calculate network throughput (bytes/second)
- Track previous network stats in MQTT client state for throughput calculation
- Update Home Assistant discovery to show RX/TX throughput sensors
- Replace cumulative byte counters with real-time throughput metrics
- Add proper throughput calculation with time-based differentials

This provides much more useful real-time network monitoring compared
to ever-increasing cumulative byte counts.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

nix/package.nix

@@ -2,6 +2,7 @@
   lib,
   beamPackages,
   src,
+  iftop,
 }:
 
 beamPackages.mixRelease rec {
@@ -10,6 +11,9 @@ beamPackages.mixRelease rec {
 
   inherit src;
 
+  # Runtime dependencies
+  buildInputs = [ iftop ];
+
   # Disable distributed Erlang to avoid COOKIE requirement
   postInstall = ''
         # Create wrapper script that sets proper environment including COOKIE