feat: bookmarks events handler partial recovery

[aintnostressin]

Prepares bookmark handlers for partial recovery.

• Reorders sync_del to read the bookmark target from the graph before deleting the edge, ensuring data survives for retries if Redis ops fail (graph-last pattern)
• Guards counter decrement behind a Redis index existence check, preventing double-decrement on retry
• Adds get_bookmark_target graph query and Bookmark::get_target_from_graph to read without side effects

[ok300]

Suggested change

	let (post_id, author_id) = match Bookmark::get_target_from_graph(&user_id, &bookmark_id).await?
	{
	Some(info) => info,
	None => return Err(EventProcessorError::SkipIndexing),
	};
	let (post_id, author_id) = Bookmark::get_target_from_graph(&user_id, &bookmark_id)
	.await?
	.ok_or(EventProcessorError::SkipIndexing)?;

[ok300]

The guard is designed to prevent double-decrement on retry, it it only handles the case where both the Redis index removal and the counter decrement succeed before the graph delete failed.

If del_from_index (step 3) succeeds, but the process crashes before UserCounts::decrement runs, then on retry existed_in_index will be false and the counter will never be decremented.

[ok300]

The test conflates the post author and the bookmarker, which means it doesn't cover the case when post author != bookmarker, which is a more realistic scenario. It would be better if the test creates 2 separate users, author_kp and bookmarker_kp.