fix(observe): render alert cadence in minutes

[rldyourmnd]

Summary

• convert second-based rule cadence to OpenObserve's supported minute frequency
• recompute consecutive-evaluation thresholds using the rendered cadence
• keep the alert lookback period at least as wide as its evaluation interval
• cover 30-second, 60-second, and 300-second contracts

Root cause

The renderer emitted evaluation_seconds: 30 as frequency: 30, frequency_type: minutes. The live OOM alert therefore evaluated every 30 minutes while querying only a five-minute delta and missed natural OOM incidents.

Validation

• red test observed against the old renderer
• make verify

Closes #182.