fix: split liveness/readiness probes, raise tolerances
- liveness: /_healthz (constant-time, no DB), 30s initial delay, 10 failures
- readiness: /_readyz (DB check), 10s initial delay, 5 failures
- prevents restart storms from blocking dependency checks
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>