Add optional GA4 page-view tracking to API home page with env-based configuration

[Copilot]

This issue asks for Google Analytics integration to capture page views and enable geography reporting, plus maintainers’ setup documentation. This PR adds GA4 instrumentation to the served home page and wires deployment/runtime configuration so tracking can be enabled in production without code changes.

• Runtime integration (API home page)

  • api/app.py now reads OSMSG_GA_MEASUREMENT_ID via get_ga_measurement_id().
  • home() passes ga_measurement_id into the template context.

• Conditional GA4 script injection

  • api/templates/home.html conditionally renders the standard gtag.js snippet only when a measurement ID is configured.
  • Default behavior remains unchanged when unset (no analytics script emitted).

• Deployment + local config plumbing

  • Added OSMSG_GA_MEASUREMENT_ID passthrough in:
    • infra/docker-compose.yml (api service)
    • docker-compose.yml (api service)
  • Added config examples/comments in:
    • .env.example
    • infra/.env.example

• Maintainer documentation

  • docs/infra.md now documents how to enable/disable GA4 via OSMSG_GA_MEASUREMENT_ID for deployed environments.

• Focused API coverage

  • tests/test_api.py adds checks for:
    • env parsing/normalization (get_ga_measurement_id)
    • script presence when configured
    • script absence when unset

def get_ga_measurement_id() -> str | None:
    measurement_id = os.getenv("OSMSG_GA_MEASUREMENT_ID", "").strip()
    return measurement_id or None

# in home handler
return Template("home.html", context={"ga_measurement_id": get_ga_measurement_id()})

• Fixes Add Google Analytics to track global page views #53