feat(coderd/database/dbpurge): make API keys retention configurable (#21037)

Replace hardcoded 7-day retention for expired API keys with configurable
retention from deployment settings. Skips deletion entirely when effective
retention is 0.

Depends on #21021
Updates #20743

coderd/database/queries/apikeys.sql

@@ -85,25 +85,20 @@ DELETE FROM
 WHERE
 	user_id = $1;
 
--- name: DeleteExpiredAPIKeys :one
+-- name: DeleteExpiredAPIKeys :execrows
 WITH expired_keys AS (
 	SELECT id
 	FROM api_keys
 	-- expired keys only
 	WHERE expires_at < @before::timestamptz
 	LIMIT @limit_count
-),
-deleted_rows AS (
-	 DELETE FROM
-		 api_keys
-	 USING
-		 expired_keys
-	 WHERE
-		 api_keys.id = expired_keys.id
-	 RETURNING api_keys.id
- )
-SELECT COUNT(deleted_rows.id) AS deleted_count FROM deleted_rows;
-;
+)
+DELETE FROM
+	api_keys
+USING
+	expired_keys
+WHERE
+	api_keys.id = expired_keys.id;
 
 -- name: ExpirePrebuildsAPIKeys :exec
 -- Firstly, collect api_keys owned by the prebuilds user that correlate