chore: implement database crud for AI seat usage (#22681)

Creates a new table `ai_seat_state` to keep track of when users consume an ai_seat. Once a user consumes an AI seat, they will forever in this table (as it stands today).

coderd/database/queries/aiseats.sql

@@ -0,0 +1,32 @@
+-- name: UpsertAISeatState :exec
+INSERT INTO ai_seat_state (
+	user_id,
+	first_used_at,
+	last_used_at,
+	last_event_type,
+	last_event_description,
+	updated_at
+)
+VALUES
+	($1, $2, $2, $3, $4, $2)
+ON CONFLICT (user_id) DO UPDATE
+SET
+	last_used_at = EXCLUDED.last_used_at,
+	last_event_type = EXCLUDED.last_event_type,
+	last_event_description = EXCLUDED.last_event_description,
+	updated_at = EXCLUDED.updated_at
+;
+
+-- name: GetActiveAISeatCount :one
+SELECT
+	COUNT(*)
+FROM
+	ai_seat_state ais
+JOIN
+	users u
+ON
+	ais.user_id = u.id
+WHERE
+	u.status = 'active'::user_status
+	AND u.deleted = false
+	AND u.is_system = false;