feat: add loaders and dbt models for Toronto housing data

Sprint 4 implementation:

Loaders:
- base.py: Session management, bulk insert, upsert utilities
- dimensions.py: Load time, district, zone, neighbourhood, policy dimensions
- trreb.py: Load TRREB purchase data to fact_purchases
- cmhc.py: Load CMHC rental data to fact_rentals

dbt Project:
- Project configuration (dbt_project.yml, packages.yml)
- Staging models for all fact and dimension tables
- Intermediate models with dimension enrichment
- Marts: purchase analysis, rental analysis, market summary

Closes #16

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

dbt/dbt_project.yml

@@ -0,0 +1,28 @@
+name: 'toronto_housing'
+version: '1.0.0'
+config-version: 2
+
+profile: 'toronto_housing'
+
+model-paths: ["models"]
+analysis-paths: ["analyses"]
+test-paths: ["tests"]
+seed-paths: ["seeds"]
+macro-paths: ["macros"]
+snapshot-paths: ["snapshots"]
+
+clean-targets:
+  - "target"
+  - "dbt_packages"
+
+models:
+  toronto_housing:
+    staging:
+      +materialized: view
+      +schema: staging
+    intermediate:
+      +materialized: view
+      +schema: intermediate
+    marts:
+      +materialized: table
+      +schema: marts