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/forgejo_error_reporter/plugin.py

@@ -23,7 +23,7 @@ class ForgejoErrorReporterPlugin(BasePlugin):
         version="1.0.0",
         display_name="Forgejo Error Reporter",
         description="Automatically reports errors to Forgejo as issues. Test environment only.",
-        is_core=False,
+        is_core=True,
         dependencies=[],
         events=[],
         migrations=[],