* MM-42282: handle teamId parameter correctly
As per https://community-daily.mattermost.com/core/pl/ugs7ue6e4j8a7cgegk1bxje8to, `ThreadStore.GetThreadsForUser` accepts a `teamId` parameter, but incorrectly handles an empty value of `""` as looking only for channels with an empty `teamId` (aka DMs and GMs) instead of finding all channels and effectively ignoring the team property.
Fixes: https://mattermost.atlassian.net/browse/MM-42282
* break up getThreadsForUser, leverage errgroup
This change breaks up `GetThreadsForUser` in the `ThreadStore` into its constituent `GetTotalUnreadThreads`, `GetTotalThreads`, `GetTotalUnreadMentions`, and the original `GetThreadsForUser` but now solely returning the thread structures. Instead of a monolithic method at the store level, the application layer now handles calling bulk requests, leveraging `errgroup` for simpler parallelization.
This change brings with it a few benefits:
* Simpler code, including more idiomatic usage of squirrel
* Simpler SQL, joining tables only when configured conditions require same. (No performance benefit here, since an unused LEFT JOIN generally has no overhead.)
* Discrete Grafana metrics for each store method, giving us better insight into the performance characteristics in play.
* **Performance boost**: reduced overhead when clearing push notifications.
This last point is what prompted the re-re-reactoring in this PR. As I broke things up, I realized that `clearPushNotificationSync` only used the `TotalUnreadMentions`, but asked for the count of total threads and total unread threads. By exposing the discrete methods, this code path avoids two aggregate queries. We clear notifications when marking a thread as read, and when marking a channel with unread mentions as viewed, so I expect we'll see at least a modest boost to performance from simply not wasting these cycles anymore.
No performance improvements are expected from this PR for the general case of using `GetThreadsForUser` to populate the threads view.
* never discard errors from building queries
* no MustSql
Co-authored-by: Mattermod <mattermod@users.noreply.github.com>
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