The per-calendar preservation added in the previous fix only kicks in when a
sync error carries a calendar_id. But account-level failures — most commonly
a Home Assistant or Google token-refresh failure — abort the entire account
fetch and arrive WITHOUT a calendar_id. Those events were still being evicted,
so HA/Google calendars kept vanishing on a transient token hiccup.
failedCalendarKeys() now returns both per-calendar keys and whole sources:
an error without a calendar_id protects every cached calendar of that source.
mergeIntoCache() keeps events whose source is in keepSourcesInRange, and the
cached range is only extended when neither set is populated (auto-retry).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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