chore: replace GetManagedAgentCount query with aggregate table (#19636)

- Removes GetManagedAgentCount query
- Adds new table `usage_events_daily` which stores aggregated usage
events by the type and UTC day
- Adds trigger to update the values in this table when a new row is
inserted into `usage_events`
- Adds a migration that adds `usage_events_daily` rows for existing data
in `usage_events`
- Adds tests for the trigger
- Adds tests for the backfill query in the migration

Since the `usage_events` table is unreleased currently, this migration
will do nothing on real deployments and will only affect preview
deployments such as dogfood.

Closes https://github.com/coder/internal/issues/943

coderd/database/models.go

@@ -3778,6 +3778,14 @@ type UsageEvent struct {
 	FailureMessage sql.NullString `db:"failure_message" json:"failure_message"`
 }
 
+// usage_events_daily is a daily rollup of usage events. It stores the total usage for each event type by day.
+type UsageEventsDaily struct {
+	// The date of the summed usage events, always in UTC.
+	Day       time.Time       `db:"day" json:"day"`
+	EventType string          `db:"event_type" json:"event_type"`
+	UsageData json.RawMessage `db:"usage_data" json:"usage_data"`
+}
+
 type User struct {
 	ID             uuid.UUID      `db:"id" json:"id"`
 	Email          string         `db:"email" json:"email"`