Add configurable normalisation threshold for alerting

[lukejudd-lux]

resolves #3921

create a local test mirror and fire off an update

testdb=# update users set name = 'Alice' where id = 1;
UPDATE 1
testdb=# select * from users;
 id |  name   |           email           |        created_at         
----+---------+---------------------------+---------------------------
  2 | Bob     | bob@example.com           | 2026-01-28 01:50:34.35945
  3 | Charlie | charlie@example.com       | 2026-01-28 01:50:34.35945
  1 | Alice   | alice.updated@example.com | 2026-01-28 01:50:34.35945
(3 rows)

wait until threshold is hit and wait for alert

Alert: Too long since last data normalize for PeerDB mirror test_mirrorData hasn't been synced to the target for mirror test_mirror since the last 15m30.791347s. This could indicate an issue with the pipeline — please check the UI and logs to confirm. Alternatively, it might be that the source database is idle and not receiving new updates. (threshold: 5 minutes)cc: ```