(feature) Add snapshot discovery action to host view

Add a staff-only POST action on host detail pages to discover existing snapshots
for that host and record them into SQL. Show success or failure feedback through
Django messages, and keep the action non-destructive before adding heavier
backup or retention controls.

Cover the action with view tests for successful discovery, redirect behavior,
and method safety.

src/pobsync_server/urls.py

@@ -9,6 +9,7 @@ from pobsync_backend import api, views
 urlpatterns = [
     path("", views.dashboard, name="dashboard"),
     path("hosts/<str:host>/", views.host_detail, name="host_detail"),
+    path("hosts/<str:host>/discover-snapshots/", views.discover_host_snapshots, name="discover_host_snapshots"),
     path("api/", api.api_index),
     path("api/status/", api.status),
     path("api/hosts/", api.hosts),