#378 fix: PodRuntimeConfig minimums default to zero instead of None

Stacked on #377.

PodRuntimeConfig.min_cpu, min_memory and min_timeout_seconds were optional, so every reader needed a None branch and the tests needed asserts just to narrow the types. A language without a minimum now carries "0" and 0. at_least always compares, timeout_for drops its or 0, and the settings loader takes the pod deadline from timeout_for instead of repeating the rule.

One behavior change: an unparseable admin limit is now rejected for every language, not only for Go. The settings schema already rejects such values at the API, so this only matters for a bad TOML default.

Ruff, mypy and unit tests pass.