fix(plugins): set is_core=True for all built-in plugins

All built-in plugins should be auto-activated. The is_core flag was
only set on some plugins, leaving Mail, DMS, Calendar, Automation,
Unified Search etc. inactive by default.

app/plugins/builtins/system_notif/plugin.py

@@ -43,7 +43,7 @@ class SystemNotifPlugin(BasePlugin):
         ],
         migrations=[],
         permissions=["system_notif:read"],
-        is_core=False,
+        is_core=True,
         settings_pages=[
             FrontendSettingsPage(path='notifications', label_key='settings.notifications', label='Notifications', component='@/pages/SettingsNotifications', icon='Bell', order=40),
         ],