feat: adopt structured runtime configuration

[lwmacct]

Summary

• replace ad-hoc runtime environment parsing with a hierarchical config manager, validation, and generated example config
• move server startup into an explicit asterrouter server command and update Docker, systemd, installer, compose, scripts, and documentation
• align CI and nightly PostgreSQL inputs with the new configuration namespace and fix the auth limiter static-analysis warning

Validation

• go test ./...
• go vet ./...
• frontend unit coverage and production build
• container, installer, single-origin, release runtime, and release browser-journey scripts
• actionlint, shellcheck, workflow pipefail check, and git diff --check

Local Playwright validation was skipped for the final workflow adjustment; CI installs Chromium and runs the browser journey.