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/figures/__init__.py

@@ -2,7 +2,6 @@
 
 from .choropleth import (
     create_choropleth_figure,
-    create_district_map,
     create_zone_map,
 )
 from .summary_cards import create_metric_card_figure, create_summary_metrics
@@ -17,7 +16,6 @@ from .time_series import (
 __all__ = [
     # Choropleth
     "create_choropleth_figure",
-    "create_district_map",
     "create_zone_map",
     # Time series
     "create_price_time_series",