state: apply a new pre-auth key's tags on re-registration

Re-registering a tagged node with a different key discarded the new key's
tags and left a stale auth-key reference; retag on key change and persist it.

Fixes #3370

Author: kradalby

CHANGELOG.md

@@ -52,6 +52,7 @@ keys remain all-access.
 ### Changes
 
 - Fix tagged node stuck expired after `tailscale logout`, unable to re-authenticate [#3394](https://github.com/juanfont/headscale/pull/3394)
+- Re-registering a tagged node with a different pre-auth key now applies the new key's tags instead of silently keeping the old ones [#3394](https://github.com/juanfont/headscale/pull/3394)
 
 ## 0.29.2 (2026-07-01)