fix(sync): add event ownership, undo-safe deletion, and ineligible task cleanup

[connectwithprakash]

Summary

• G3 fix: Clean up orphaned calendar events when a task becomes ineligible for sync (date/time/duration removed)
• G7 fix: Defer calendar event deletion to after the 5-second undo window so undo fully restores both task and event
• G9 feature: Add isEventOwner flag — tasks created from calendar events never modify or delete the source event on complete/delete

Details

Event Ownership (isEventOwner)

• New Boolean attribute on TaskEntity (default true, lightweight migration)
• createTaskFromEvent / createTaskFromCalendarEvent set it to false
• handleCompletedTask, pushUpdatesToEvent, handleIneligibleTask all guard on ownership
• deleteTask resolves deleteLinkedEvent to task.isEventOwner when not explicitly passed

Undo-Safe Deletion

• Soft-delete no longer immediately deletes the linked calendar event
• Event deletion deferred to commitPendingChanges() (after undo window closes)
• discardPendingChanges() (undo) works fully — event was never deleted

Ineligible Task Cleanup

• New handleIneligibleTask method in performForwardSync
• Catches linked tasks that lost date/time/duration eligibility
• Deletes orphaned event and unlinks the task

Test plan

• Create task with date+time+duration → verify event created in Lazyflow calendar
• Remove date from linked task → verify event deleted and task unlinked
• Swipe-delete linked task → verify event still exists during 5s undo window
• Tap undo → verify both task and event restored
• Let undo window expire → verify event deleted
• Create task from calendar event → complete it → verify source event untouched
• Delete task created from event → verify source event untouched

Closes #283

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