Add long-term forecast horizon support to skill metrics

[mabesa]

Summary

• Add VALID_SKILL_METRIC_HORIZONS constant as the union of VALID_HORIZONS and VALID_LONG_FORECAST_HORIZONS, enabling skill metrics to accept
  "quarter" and all long-term forecast horizons
• Update read_skill_metrics() validation and prepare_skill_metric_records() Literal type to use the new set
• Export VALID_SKILL_METRIC_HORIZONS from the package public API
• Preprocessing and short-term clients remain unaffected

Test plan

• 188 tests pass, 0 skipped
• mypy passes with no issues
• New tests verify "quarter" is accepted by skill metrics read/prepare
• Regression guard: "quarter" is still rejected by preprocessing
• Constant tests verify the union relationship between horizon sets