Honor start_period grace before the readiness rollout deadline

[Shine-neko]

What

Readiness health checks now accept an optional start_period grace window. Some containers legitimately fail readiness for their whole boot because they build their app after starting (e.g. bun run build behind Caddy). Without a grace, a short RING_ROLLOUT_DEADLINE fails a perfectly healthy slow-building app the instant its build outlasts the deadline.

With start_period, the rollout deadline only starts counting after the grace window. The effective budget becomes start_period + RING_ROLLOUT_DEADLINE. For command checks on Docker, the value is also forwarded to the native HEALTHCHECK start_period.

health_checks:
  - type: command
    command: test -f /var/run/kemeter/ready
    interval: 5s
    timeout: 2s
    threshold: 3
    on_failure: alert
    readiness: true
    start_period: "180s"     # 3 min of build/boot before failures count

• Per-check; the scheduler takes the maximum across readiness checks.
• A malformed value logs a warning and is ignored (grace falls back to zero rather than blocking a slow build).
• Ignored when readiness: false. Same duration syntax as interval / timeout.

Bug caught by e2e

The apply client re-declares its own HealthCheck struct to parse YAML, and it was missing the new field — so start_period was silently dropped before ever reaching the API, leaving the feature inert. Cargo tests build the deployment in memory and never exercise the apply → API → DB round-trip, so they passed while the feature did nothing. The e2e (which drives the real binary) surfaced it: the stored health_checks JSON had no start_period, and a deployment with start_period: 40s still failed at the bare 10s deadline. Fixed by adding the field to the apply-side struct.

Tests

• tests/e2e/docker/t37_readiness_start_period.sh (new): with a never-ready probe and a short deadline, proves a no-grace deployment fails at the deadline, a start_period: 40s deployment survives well past it, and the deferred safety valve still fires once the budget elapses.
• 730 cargo tests pass; t22_readiness_rolling_update.sh still green (no regression on the readiness gate).