…st practices

- Update Go OTel SDK from v1.18.0 to v1.40.0 (latest stable)
- Update otelhttp from v0.44.0 to v0.65.0
- Update Go version from 1.20 to 1.24
- Change metric instruments from UpDownCounter to Gauge (correct OTel
  instrument for point-in-time container resource measurements)
- Add metric descriptions and units per OTel semantic conventions
- Add proper error handling throughout (return errors instead of
  swallowing them)
- Add graceful shutdown with signal handling (SIGINT/SIGTERM)
- Add HTTP server timeouts (read/write/idle)
- Add /health endpoint for basic health checks
- Add startup probe in Kubernetes manifest
- Use multi-stage Docker build with distroless base image
- Add pod and container security context (non-root, read-only FS,
  drop all capabilities)
- Right-size resource requests/limits (100m/128Mi to 500m/256Mi)
- Add comments documenting Datadog metric name mapping
- Add OTel resource creation with semconv service.name
- Remove unused go.uber.org/zap dependency
- Document environment variables, endpoints, and metrics in README

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>