Adds Prometheus instrumentation across the checkout funnel, auth flow,
Stripe calls, rate limiters, and HTTP middleware (inflight gauge, request
and response sizes). Introduces a slog handler that counts log lines by
level so the dashboard can show error/warn rates; existing log.Printf
calls are routed through slog and counted as info.
Backs the new "terminalshop — Service Overview" Grafana dashboard.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
