mergeIntoCache() unconditionally evicted all cached events in the fetched
range, so a transient sync failure (expired CalDAV credentials, an HA/Google
token-refresh failure) wiped the affected calendar until the next clean fetch.
Home Assistant was the most visible victim: a token failure aborts the whole
account fetch and the error arrives without a calendar_id.
Now sync errors are split into per-calendar keys (error carries a calendar_id)
and whole sources (account-level error without one). mergeIntoCache() keeps
matching cached events, and only extends the cached range on a fully clean
fetch so failed calendars are retried on the next load. Mirrors the iOS fix.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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