fix(installer): point dashcaddy_auth snippet at /api/v1/auth/gate

The 1.5.0 API cutover removed the legacy `/api` mount, but the
`dashcaddy_auth` Caddy snippet generated by the installer still called
`uri /api/auth/gate/{args[0]}`. forward_auth then hit the notFoundHandler
and propagated a DC-404 to the browser instead of a 401-redirect to the
login flow (seen on requests.sami).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

dashcaddy-installer/install.sh

@@ -539,7 +539,7 @@ SNIP
 
   local auth_snippet="(dashcaddy_auth) {
   forward_auth localhost:${API_PORT} {
-    uri /api/auth/gate/{args[0]}
+    uri /api/v1/auth/gate/{args[0]}
     copy_headers Authorization X-Api-Key X-App-Cookie X-Emby-Token X-Plex-Token
   }
 }"