feat: add comprehensive health check endpoints

[PolNavarro]

Summary

• Added comprehensive health check system with 3 endpoints: /health, /health/ready, and /health/live
• Implemented detailed system monitoring including webhook validation, memory checks, and filesystem access
• Added proper HTTP status codes for different health states (200/503)
• Health endpoints bypass authentication for monitoring tools

New Endpoints

/health - Complete Health Check

Returns detailed JSON with:

• Overall system status (healthy/degraded/unhealthy)
• Individual component checks (configuration, memory, filesystem, webhooks)
• System information (version, uptime, Rust version)
• Webhook configuration validation

/health/ready - Readiness Check

• Returns 200 if service is ready to handle requests
• Returns 503 if not ready (no webhooks configured)

/health/live - Liveness Check

• Simple 200 response if service is alive

Implementation Details

• Non-blocking: All checks are designed to be fast
• No authentication: Health endpoints are public for monitoring
• Detailed validation: Checks webhook paths, script files, configuration integrity
• Proper error handling: Different HTTP status codes based on health state

Testing

# Start server
cargo run -- -c examples/health_test.toml

# Test endpoints
curl http://localhost:8080/health
curl http://localhost:8080/health/ready
curl http://localhost:8080/health/live

Example Response

{
  "status": "healthy",
  "timestamp": 1691234567,
  "version": "0.2.2",
  "uptime_seconds": 3600,
  "checks": {
    "configuration": {
      "status": "ok",
      "message": "2 webhooks configured",
      "details": {
        "webhook_count": 2,
        "webhook_paths": ["deploy", "test"],
        "port": 8080
      }
    },
    "webhooks": {
      "status": "ok",
      "message": "All webhooks configured correctly"
    }
  }
}

Test plan

• Compile and run the server with example configuration
• Test /health endpoint returns detailed JSON
• Test /health/ready returns 200 when webhooks configured
• Test /health/live always returns 200
• Verify health endpoints work without authentication
• Test with invalid configurations to ensure proper error states
• Verify existing webhook functionality remains unchanged

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