feat: add reference data loading and filter state callbacks (Task 3.1)

dash_app/callbacks/__init__.py

@@ -0,0 +1,8 @@
+"""Callback registration — imports all callback modules and wires them to the app."""
+
+
+def register_callbacks(app):
+    """Register all Dash callbacks with the app instance."""
+    from dash_app.callbacks.filters import register_filter_callbacks
+
+    register_filter_callbacks(app)