Implements Phases 02 (partial) and 03 of the go-port-cloud-run plan.
Introduces module framework with per-module KV prefix isolation,
health check endpoint, request timeout protection, and comprehensive
test coverage. Cloud Run deployment deferred to Phase 01.
Security hardening: constant-time secret comparison, cron auth bridge,
and secrets stripped from dependency environment exports. Includes
Dockerfile, GitHub CI workflow (vet + race + build), and integration
tests for module lifecycle.
