fix(notifications): record a relay refusal at warn, not error

A 502 from a channel test reports the user's relay or mail setup, not a fault on this server, yet the status alone raised every press to an error-level event. The event builder gains a ceiling for the status-derived level, and the test routes cap it at warn on the 502 path. The 500 for an unnamed failure stays at error, and explicit elevations are unaffected.

The notifications guide notes that a reverse proxy which replaces 5xx pages hides the detail, so the card falls back to Test failed.

Refs #947

docs/self-hosting/notifications.md

@@ -274,6 +274,13 @@ short, Gotify's own error text. A `401` means the token is wrong. A `400`
 means Gotify could not read the body; check that the format is set to
 Gotify. A Gotify on your own network also needs the operator grant below.
 
+The detail reaches the card inside the server's `502` response. A reverse
+proxy that replaces error pages for 5xx responses, such as Traefik's
+`errors` middleware or nginx with `proxy_intercept_errors on`, swaps that
+response for its own page, and the card can then only say "Test failed".
+Exclude the `/api/` paths from the error-page rule to keep the detail. This
+applies to the ntfy and email tests too.
+
 ## Notification targets on a private network
 
 Every webhook and ntfy target is resolved at send time and refused when