feat: configurable history window (fixed|rolling) + offset hours (#655)

[Sagarpillai]

Implements #655.

Adds a configurable history window for start_datetime_utc in generation + forecast queries.

Two env vars control this: • NEXT_PUBLIC_HISTORY_START_TYPE = “rolling” or “fixed” • NEXT_PUBLIC_HISTORY_START_OFFSET_HOURS = numeric hour offset (default 48)

“rolling” = now - offset hours (existing behaviour) “fixed” = midnight UTC N days ago

getHistoryStartISO() now reads these envs, and both queries use it.

This allows Vercel preview / production to tune the history window without code changes.

Local build + typecheck pass. Defaults keep current behaviour.

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[Sagarpillai]

Hey, @braddf Would love a review of this whenever you get a chance. Thank you in advance!