feat(alerts): add scheduleStartAt + scheduleOffsetMinutes

[mlsalcedo]

Closes #1715

Summary

• Add scheduleStartAt and scheduleOffsetMinutes to alert schemas and API validation.
• Update alert evaluation scheduling to anchor windows by scheduleStartAt when set.
• Skip evaluations before scheduleStartAt.
• Keep current behavior unchanged when scheduling fields are unset.
• Add UI fields and API/OpenAPI/external API support for the new schedule options.
• Add alert scheduler tests for anchored windows and pre-start skip behavior.

Notes

• This enables Splunk-style scheduled monitor migration where checks must run on isolated, periodic windows anchored to specific times.
• scheduleStartAt is the primary anchor; scheduleOffsetMinutes remains optional for backward-compatible alignment.

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Latest commit: 2d29bb5

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
@hyperdx/api	Minor
@hyperdx/app	Minor
@hyperdx/common-utils	Minor
@hyperdx/otel-collector	Minor

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[sgarfinkel]

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[github-actions]

PR Review: feat(alerts): add scheduleStartAt + scheduleOffsetMinutes

No critical issues found. Implementation is well-structured with solid test coverage.

A few items worth noting:

• WARNING: validateAlertScheduleOffsetMinutes skips offset validation when scheduleStartAt is set — The check scheduleOffsetMinutes >= intervalMinutes still runs even when scheduleStartAt is provided (though it correctly does nothing at that point since offset is forced to 0). Not a bug, just a minor clarity issue.

• WARNING: Date range limits use approximate year lengths (365 * 24 * 60 * 60 * 1000) — won't account for leap years. Negligible in practice but worth noting.

• INFO: Redundant max(1439) on base Zod schema in zod.ts and types.ts — the superRefine already enforces the dynamic per-interval max. Not harmful, just redundant.

• INFO: makeAlert calls new Date(alert.scheduleStartAt) without error handling — safe because alertSchema.parse() is now called before makeAlert in all routers, ensuring scheduleStartAt is a valid ISO string. The recent change to explicitly parse in routers (previously req.body was used directly) is a good fix.

The backward-compat strategy via normalizeNoOpAlertScheduleFields and the null-vs-undefined distinction for scheduleStartAt serialization are well thought out.

Reviewed by Claude Sonnet 4.6