chore: Remove TRREB references from Python modules

- Remove DimTRREBDistrict model and FactPurchases model
- Remove TRREBDistrict schema and AreaType enum
- Remove TRREBDistrictParser from geo parsers
- Remove load_trreb_districts from dimension loaders
- Remove create_district_map from choropleth figures
- Remove get_demo_districts and get_demo_purchase_data from demo_data
- Update summary metrics to remove purchase-related metrics
- Update callbacks to remove TRREB-related comments
- Update methodology page to remove TRREB data source section
- Update dashboard data notice to remove TRREB mention

Closes #49

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

portfolio_app/pages/toronto/dashboard.py

@@ -257,9 +257,8 @@ def create_data_notice() -> dmc.Alert:
     return dmc.Alert(
         children=[
             dmc.Text(
-                "This dashboard uses TRREB and CMHC data. "
-                "Geographic boundaries require QGIS digitization to enable choropleth maps. "
-                "Sample data is shown below.",
+                "This dashboard displays Toronto neighbourhood and CMHC rental data. "
+                "Sample data is shown for demonstration purposes.",
                 size="sm",
             ),
         ],