_stable_cal_id hashed only the remote URL, producing the same calendar
ID for all users syncing the same CalDAV endpoint. The second user would
get an IntegrityError on the primary key. Now includes owner in the
hash so each user gets a distinct calendar row.
The CalDAV pull prunes events in the synced calendar+window whose UID the
server didn't just return, to propagate upstream deletions. But CalendarEvent
had no field distinguishing a server-pulled row from a locally-created one, so
the prune also deleted events that were never on the server: events created by
the agent / email triage (which never write back to the server) and UI events
whose best-effort write-back failed. Result: silent, unrecoverable loss of the
user's appointments (hard db.delete, no soft-delete).
Add an 'origin' column to calendar_events (lightweight idempotent migration,
mirroring _migrate_add_calendar_is_utc), set origin='caldav' on rows the sync
inserts/updates, and gate the prune on origin == 'caldav'. Locally-created
events carry origin NULL and are never pruned. On the first sync after the
migration nothing is pruned (all rows NULL until re-marked), erring toward
keeping data.
Fixes#2704
Ernest Hysa