Phase 2: Tags UI, Custom Fields UI, Notifications Bell

- Tags UI: TagsPage (CRUD, color picker), TagBadge, TagSelector (multi-select, inline creation)
- Custom Fields Backend: model, schema, service, routes, migration 0041
- Custom Fields Frontend: CustomFieldsPage (definitions CRUD), CustomFieldRenderer (dynamic field rendering)
- Custom Fields: _collect_custom_field_definitions() extended to merge DB definitions with plugin definitions
- Notifications Bell: NotificationBell (30s polling, unread badge), NotificationDropdown, NotificationItem
- NotificationBell integrated into TopBar
- Routes: /tags, /settings/custom-fields registered
- Settings nav: Custom Fields entry added
- Menu items: Tags added to automation plugin manifest

app/plugins/builtins/automation/plugin.py

@@ -92,6 +92,13 @@ class AutomationPlugin(BasePlugin):
                 icon="Copy",
                 order=54,
             ),
+            FrontendMenuItem(
+                label_key="nav.tags",
+                label="Tags",
+                path="/tags",
+                icon="Tag",
+                order=55,
+            ),
         ],
         page_routes=[
             FrontendPageRoute(