break up getThreadsForUser, leverage errgroup (#19709)

* 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>

app/notification_push.go

@@ -236,11 +236,11 @@ func (a *App) clearPushNotificationSync(currentSessionId, userID, channelID, roo
 	msg.Badge = int(unreadCount)
 
 	if msg.IsCRTEnabled {
-		data, err := a.Srv().Store.Thread().GetThreadsForUser(userID, "", model.GetUserThreadsOpts{TotalsOnly: true})
+		totalUnreadMentions, err := a.Srv().Store.Thread().GetTotalUnreadMentions(userID, "", model.GetUserThreadsOpts{})
 		if err != nil {
 			return model.NewAppError("clearPushNotificationSync", "app.user.get_thread_count_for_user.app_error", nil, err.Error(), http.StatusInternalServerError)
 		}
-		msg.Badge += int(data.TotalUnreadMentions)
+		msg.Badge += int(totalUnreadMentions)
 	}
 	return a.sendPushNotificationToAllSessions(msg, userID, currentSessionId)
 }