Normalize expiry calculation to UTC with fixed 24h days

[mkysel]

Previously expiry times were computed using time.Now().AddDate(...), which
advances calendar days in local time. This caused expiry drift of ±1h when
crossing DST boundaries (e.g. Nov 2, 2025 in America/New_York), leading to
test failures and inconsistent retention windows.

We now calculate expiry as:

time.Now().UTC().Add(time.Hour * 24 * retentionDays)

This normalizes to UTC and ensures each "day" of retention is treated as an
exact 24-hour period, independent of daylight saving time changes or local
timezone shifts.

Tests updated accordingly to use the same UTC 24h-day semantics.

[graphite-app]

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[macroscopeapp]

Normalize expiry timestamp calculation to UTC with fixed 24-hour increments in migrator.Transformer.buildAndSignOriginatorEnvelope and registrant.Registrant.SignStagedEnvelope

This change updates expiry calculations to use UTC with exact 24-hour day increments and improves test assertion output for readability.

• Update ExpiryUnixtime to time.Now().UTC().Add(time.Hour*24*time.Duration(retentionDays)).Unix() in migrator.Transformer.buildAndSignOriginatorEnvelope in transformer.go
• Update ExpiryUnixtime to time.Now().UTC().Add(time.Hour*24*time.Duration(retentionDays)).Unix() in registrant.Registrant.SignStagedEnvelope in registrant.go
• Improve require.InDelta message to show human-readable expected/actual timestamps in payer.TestPublishToNodes in publish_test.go

📍Where to Start

Start with the expiry computation in migrator.Transformer.buildAndSignOriginatorEnvelope in transformer.go, then review the corresponding change in registrant.Registrant.SignStagedEnvelope in registrant.go.

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Macroscope summarized 594d074.