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/profiles.yml.example

@@ -0,0 +1,21 @@
+toronto_housing:
+  target: dev
+  outputs:
+    dev:
+      type: postgres
+      host: localhost
+      user: portfolio
+      password: "{{ env_var('POSTGRES_PASSWORD') }}"
+      port: 5432
+      dbname: portfolio
+      schema: public
+      threads: 4
+    prod:
+      type: postgres
+      host: "{{ env_var('POSTGRES_HOST') }}"
+      user: "{{ env_var('POSTGRES_USER') }}"
+      password: "{{ env_var('POSTGRES_PASSWORD') }}"
+      port: 5432
+      dbname: portfolio
+      schema: public
+      threads: 4