Adds HTTP health check server for remote monitoring from @aecs4u (PR #21).

## Endpoints

### GET /health
Simple health check returning 200 OK if gateway is running.
Useful for load balancer health checks.

### GET /status
Detailed JSON status with metrics:
```json
{
  "status": "healthy",
  "uptime_secs": 3600,
  "total_connections": 42,
  "active_connections": 3,
  "total_messages": 1234
}
```

## Features

### HealthStats
Shared statistics tracked across connections:
- Start time (for uptime calculation)
- Total connections (cumulative)
- Active connections (current)
- Total messages processed

### Usage
```rust
use rustyclaw::gateway::health::{start_health_server, HealthStats, SharedHealthStats};

let stats = Arc::new(HealthStats::new());

// Start health server on separate port
tokio::spawn(start_health_server(
    "127.0.0.1:8081",
    stats.clone(),
    cancel_token,
));

// Increment stats in connection handler
stats.total_connections.fetch_add(1, Ordering::Relaxed);
stats.active_connections.fetch_add(1, Ordering::Relaxed);
```

## Use Cases
- Load balancer health checks
- Uptime monitoring (Pingdom, UptimeRobot)
- Metrics collection
- Remote status inspection

## No New Dependencies
Uses existing: tokio, serde_json, anyhow

## Attribution
Original implementation by @aecs4u