feat(platform): bootstrap local observability stack

[capybaracplusplus]

Summary

• add a loopback-bound local Compose stack for PostgreSQL, auth, Mailpit, Prometheus, and Grafana
• provision Prometheus scrape configuration, auth outbox alerts, and a Grafana dashboard
• generate ignored local JWT keys and runtime config through an idempotent init script
• pin third-party images by version and multi-arch digest
• validate Compose, Prometheus, Grafana JSON, and shell configuration in CI

Dependency

• Expose email outbox observability metrics smirkly-auth#39 exposes the custom auth outbox metrics consumed here

Validation

• bash -n scripts/init-local.sh
• init script executed twice successfully
• Grafana dashboard JSON parsed successfully
• all tracked YAML parsed successfully
• git diff --cached --check

Runtime Compose validation is intentionally left for Docker Desktop and CI because the auth devcontainer does not contain the Docker CLI.