FIX: Make summaries backfill job more resilient.

[romanrizzi]

To quickly select backfill candidates without comparing SHAs, we compare the last summarized post to the topic's highest_post_number. However, hiding or deleting a post and adding a small action will update this column, causing the job to stall and re-generate the same summary repeatedly until someone posts a regular reply. On top of this, this is not always true for topics with best_replies, as this last reply isn't necessarily included.

Since this is not evident at first glance and each summarization strategy picks its targets differently, I'm opting to simplify the backfill logic and how we track potential candidates.

The first step is dropping content_range, which serves no purpose and it's there because summary caching was supposed to work differently at the beginning. So instead, I'm replacing it with a column called highest_target_number, which tracks highest_post_number for topics and could track other things like channel's message_count in the future.

Now that we have this column when selecting every potential backfill candidate, we'll check if the summary is truly outdated by comparing the SHAs, and if it's not, we just update the column and move on

[SamSaffron]

Generally I think this is OK, one challenge is going to be stuff like documentation topics, where there is huge amounts of churn on the topic being summarized but backfill does not pick up on it.

I would go ahead and merge this to stop the bleeding but long term we may need to think about this problem