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/unique_constraint.go

@@ -67,6 +67,7 @@ const (
 	UniqueTemplateVersionsPkey                                UniqueConstraint = "template_versions_pkey"                                          // ALTER TABLE ONLY template_versions ADD CONSTRAINT template_versions_pkey PRIMARY KEY (id);
 	UniqueTemplateVersionsTemplateIDNameKey                   UniqueConstraint = "template_versions_template_id_name_key"                          // ALTER TABLE ONLY template_versions ADD CONSTRAINT template_versions_template_id_name_key UNIQUE (template_id, name);
 	UniqueTemplatesPkey                                       UniqueConstraint = "templates_pkey"                                                  // ALTER TABLE ONLY templates ADD CONSTRAINT templates_pkey PRIMARY KEY (id);
+	UniqueUsageEventsDailyPkey                                UniqueConstraint = "usage_events_daily_pkey"                                         // ALTER TABLE ONLY usage_events_daily ADD CONSTRAINT usage_events_daily_pkey PRIMARY KEY (day, event_type);
 	UniqueUsageEventsPkey                                     UniqueConstraint = "usage_events_pkey"                                               // ALTER TABLE ONLY usage_events ADD CONSTRAINT usage_events_pkey PRIMARY KEY (id);
 	UniqueUserConfigsPkey                                     UniqueConstraint = "user_configs_pkey"                                               // ALTER TABLE ONLY user_configs ADD CONSTRAINT user_configs_pkey PRIMARY KEY (user_id, key);
 	UniqueUserDeletedPkey                                     UniqueConstraint = "user_deleted_pkey"                                               // ALTER TABLE ONLY user_deleted ADD CONSTRAINT user_deleted_pkey PRIMARY KEY (id);