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/unified_search/plugin.py

@@ -39,7 +39,7 @@ class UnifiedSearchPlugin(BasePlugin):
         ],
         migrations=["0001_initial.sql", "0002_embeddings.sql", "0003_add_deleted_at.sql"],
         permissions=["search:read", "search:admin"],
-        is_core=False,
+        is_core=True,
         page_routes=[
             FrontendPageRoute(path='/search', component='@/pages/GlobalSearchResults', protected=True),
         ],