fix(appsettings,vrtManager,storages): error handlers that hid the real failure

Reviewing the error handlers outside instances/ turned up three that masked or
swallowed the failure they caught:

- appsettings/settings.get_settings() runs on every request (middleware) and
  loads app_settings from the database. Its bare `except: pass` left `entries`
  unbound on a database error, so the next line raised UnboundLocalError
  instead; it also caught KeyboardInterrupt/SystemExit. It now catches
  DatabaseError and returns, keeping the already-loaded values.
- vrtManager.network.refresh_dhcp_leases raised a formatted string
  (`raise f"..."`), which Python rejects with TypeError, hiding the real
  DHCPLeases failure. It now raises util.OperationError, which the API/HTML
  error handling already renders as a libvirt error, and clears self.leases.
- storages.views.get_volumes caught libvirtError and answered 200 with an empty
  body, so the caller could not tell a failed pool read from an empty pool. It
  now returns 400 with the error.

The other broad handlers reviewed (appsettings/nwfilters/virtsecrets views,
storages/upload cleanup and recovery paths, vrtManager display getters) report
the error or are best-effort and were left as they are.

Tests: get_settings() survives a database error during the query and during
iteration without raising and still loads a real value; refresh_dhcp_leases
raises OperationError (keeping the cause) not TypeError; get_volumes returns 400
on a listing failure and the sorted volumes on success. The tests fail on the
previous code.

appsettings/settings.py

@@ -1,3 +1,5 @@
+from django.db import DatabaseError
+
 from .models import AppSettings
 
 
@@ -10,9 +12,8 @@ app_settings = Settings()
 
 def get_settings():
     try:
-        entries = AppSettings.objects.all()
-    except:
-        pass
-
+        entries = list(AppSettings.objects.all())
+    except DatabaseError:
+        return  # keep the values already loaded; a failed read must not crash the request
     for entry in entries:
         setattr(app_settings, entry.key, entry.value)