feat: add notification for task status (#19965)

## Description

Send a notification to the workspace owner when an AI task’s app state
becomes `Working` or `Idle`.
An AI task is identified by a workspace build with `HasAITask = true`
and `AITaskSidebarAppID` matching the agent app’s ID.

## Changes

* Add `TemplateTaskWorking` notification template.
* Add `TemplateTaskIdle` notification template.
* Add `GetLatestWorkspaceAppStatusesByAppID` SQL query to get the
workspace app statuses ordered by latest first.
* Update `PATCH /workspaceagents/me/app-status` to enqueue:
  * `TemplateTaskWorking` when state transitions to `working`
  * `TemplateTaskIdle` when state transitions to `idle`
* Notification labels include:
  * `task`: task initial prompt
  * `workspace`: workspace name
* Notification dedupe: include a minute-bucketed timestamp (UTC
truncated to the minute) in the enqueue data to allow identical content
to resend within the same day (but not more than once per minute).

Closes: https://github.com/coder/coder/issues/19776

coderd/database/dbmetrics/querymetrics.go

@@ -985,6 +985,13 @@ func (m queryMetricsStore) GetLatestCryptoKeyByFeature(ctx context.Context, feat
 	return r0, r1
 }
 
+func (m queryMetricsStore) GetLatestWorkspaceAppStatusesByAppID(ctx context.Context, appID uuid.UUID) ([]database.WorkspaceAppStatus, error) {
+	start := time.Now()
+	r0, r1 := m.s.GetLatestWorkspaceAppStatusesByAppID(ctx, appID)
+	m.queryLatencies.WithLabelValues("GetLatestWorkspaceAppStatusesByAppID").Observe(time.Since(start).Seconds())
+	return r0, r1
+}
+
 func (m queryMetricsStore) GetLatestWorkspaceAppStatusesByWorkspaceIDs(ctx context.Context, ids []uuid.UUID) ([]database.WorkspaceAppStatus, error) {
 	start := time.Now()
 	r0, r1 := m.s.GetLatestWorkspaceAppStatusesByWorkspaceIDs(ctx, ids)