chore: Delete legacy TRREB dbt models

- Delete stg_trreb__purchases.sql and stg_dimensions__trreb_districts.sql
- Delete int_purchases__monthly.sql
- Delete mart_toronto_purchases.sql and mart_toronto_market_summary.sql
- Update _sources.yml to remove fact_purchases and dim_trreb_district
- Update _staging.yml to remove TRREB staging models
- Update _intermediate.yml to remove int_purchases__monthly
- Update _marts.yml to remove purchase-related marts

Closes #48

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

dbt/models/marts/_marts.yml

@@ -1,15 +1,6 @@
 version: 2
 
 models:
-  - name: mart_toronto_purchases
-    description: "Final mart for Toronto purchase/sales analysis by district and time"
-    columns:
-      - name: purchase_id
-        description: "Unique purchase record identifier"
-        tests:
-          - unique
-          - not_null
-
   - name: mart_toronto_rentals
     description: "Final mart for Toronto rental market analysis by zone and time"
     columns:
@@ -18,6 +9,3 @@ models:
         tests:
           - unique
           - not_null
-
-  - name: mart_toronto_market_summary
-    description: "Combined market summary aggregating purchases and rentals at Toronto level"