iOS Reminders (and other RFC 6578 clients) never learned about tasks
deleted on the Vikunja side: caldav-go answers unknown REPORT types
with 412, which makes iOS silently stop syncing, and deleted tasks left
no protocol-level trace a client could observe.
Intercept sync-collection REPORTs before caldav-go sees them and answer
per RFC 6578: tasks changed since the sync token as 200 entries and
deleted tasks as 404 entries. Deletion records come from task
soft-deletes, which are kept for 30 days before being purged.
Token edge cases: delta comparisons are inclusive because tokens have
second granularity (re-reported items keep their etag, so clients skip
the download), and tokens older than the soft-delete retention answer
403 + valid-sync-token to force a full resync instead of a delta that
would silently miss purged deletions.
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