Show the Name servers check as it happens, and keep its results

The zone's Name servers tab no longer waits for the other servers before
showing: it lists each query as a dig command line with a waiting
counter, fills in the answers as they arrive, then shows the result with
the queries under "Queries sent". Results (serials and delegation) are
kept for 10 minutes so revisiting the tab is instant; "Check again" asks
anew, and a check keeps running when the tab is left.

GET /zones/{zone}/serials?stream=1 streams the queries as
newline-delimited JSON (nscheck.Checker.Watch); without it the endpoint
is unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -104,6 +104,15 @@ All notable changes to Jarl are documented here. The format follows
 - The view in the zone list, and on the zone page, links to that view,
   and the views page highlights it. It used to be plain text, and the zone
   page linked to the views page as a whole.
+- The zone's Name servers tab shows at once and follows the check as it
+  happens. Each query is listed as a dig command line (`dig @192.0.2.1
+  example.org. SOA +norec`) with a "waiting 1.8 s" counter until it is
+  answered. The finished result then replaces the progress, and the queries
+  stay under "Queries sent". Results, the delegation check's included, are
+  kept for 10 minutes, so returning to the tab shows them at once; "Check
+  again" asks anew. A check keeps running if you leave the tab. Behind it,
+  `GET /zones/{zone}/serials?stream=1` streams the queries as
+  newline-delimited JSON (docs/api.md).
 - Switching a zone's tabs (Records, Settings, …) no longer blanks the page
   while the tab loads: the page changes once the tab is ready, and
   "Loading…" appears only if a page takes a while.