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>

store/timerlayer/timerlayer.go

@@ -8455,7 +8455,7 @@ func (s *TimerLayerThreadStore) GetThreadUnreadReplyCount(threadMembership *mode
 	return result, err
 }
 
-func (s *TimerLayerThreadStore) GetThreadsForUser(userId string, teamID string, opts model.GetUserThreadsOpts) (*model.Threads, error) {
+func (s *TimerLayerThreadStore) GetThreadsForUser(userId string, teamID string, opts model.GetUserThreadsOpts) ([]*model.ThreadResponse, error) {
 	start := timemodule.Now()
 
 	result, err := s.ThreadStore.GetThreadsForUser(userId, teamID, opts)
@@ -8471,6 +8471,54 @@ func (s *TimerLayerThreadStore) GetThreadsForUser(userId string, teamID string,
 	return result, err
 }
 
+func (s *TimerLayerThreadStore) GetTotalThreads(userId string, teamID string, opts model.GetUserThreadsOpts) (int64, error) {
+	start := timemodule.Now()
+
+	result, err := s.ThreadStore.GetTotalThreads(userId, teamID, opts)
+
+	elapsed := float64(timemodule.Since(start)) / float64(timemodule.Second)
+	if s.Root.Metrics != nil {
+		success := "false"
+		if err == nil {
+			success = "true"
+		}
+		s.Root.Metrics.ObserveStoreMethodDuration("ThreadStore.GetTotalThreads", success, elapsed)
+	}
+	return result, err
+}
+
+func (s *TimerLayerThreadStore) GetTotalUnreadMentions(userId string, teamID string, opts model.GetUserThreadsOpts) (int64, error) {
+	start := timemodule.Now()
+
+	result, err := s.ThreadStore.GetTotalUnreadMentions(userId, teamID, opts)
+
+	elapsed := float64(timemodule.Since(start)) / float64(timemodule.Second)
+	if s.Root.Metrics != nil {
+		success := "false"
+		if err == nil {
+			success = "true"
+		}
+		s.Root.Metrics.ObserveStoreMethodDuration("ThreadStore.GetTotalUnreadMentions", success, elapsed)
+	}
+	return result, err
+}
+
+func (s *TimerLayerThreadStore) GetTotalUnreadThreads(userId string, teamID string, opts model.GetUserThreadsOpts) (int64, error) {
+	start := timemodule.Now()
+
+	result, err := s.ThreadStore.GetTotalUnreadThreads(userId, teamID, opts)
+
+	elapsed := float64(timemodule.Since(start)) / float64(timemodule.Second)
+	if s.Root.Metrics != nil {
+		success := "false"
+		if err == nil {
+			success = "true"
+		}
+		s.Root.Metrics.ObserveStoreMethodDuration("ThreadStore.GetTotalUnreadThreads", success, elapsed)
+	}
+	return result, err
+}
+
 func (s *TimerLayerThreadStore) MaintainMembership(userID string, postID string, opts store.ThreadMembershipOpts) (*model.ThreadMembership, error) {
 	start := timemodule.Now()