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/models/marts/mart_toronto_rentals.sql

@@ -0,0 +1,64 @@
+-- Mart: Toronto Rental Market Analysis
+-- Final analytical table for rental market visualization
+-- Grain: One row per zone per bedroom type per survey year
+
+with rentals as (
+    select * from {{ ref('int_rentals__annual') }}
+),
+
+-- Add year-over-year calculations
+with_yoy as (
+    select
+        r.*,
+
+        -- Previous year values
+        lag(r.avg_rent, 1) over (
+            partition by r.zone_code, r.bedroom_type
+            order by r.year
+        ) as avg_rent_prev_year,
+
+        lag(r.vacancy_rate, 1) over (
+            partition by r.zone_code, r.bedroom_type
+            order by r.year
+        ) as vacancy_rate_prev_year
+
+    from rentals r
+),
+
+final as (
+    select
+        rental_id,
+        date_key,
+        full_date,
+        year,
+        quarter,
+        zone_key,
+        zone_code,
+        zone_name,
+        bedroom_type,
+        rental_universe,
+        avg_rent,
+        median_rent,
+        vacancy_rate,
+        availability_rate,
+        turnover_rate,
+        year_over_year_rent_change,
+        reliability_code,
+        vacant_units_estimate,
+
+        -- Calculated year-over-year (if not provided)
+        coalesce(
+            year_over_year_rent_change,
+            case
+                when avg_rent_prev_year > 0
+                then round(((avg_rent - avg_rent_prev_year) / avg_rent_prev_year) * 100, 2)
+                else null
+            end
+        ) as rent_change_pct,
+
+        vacancy_rate - vacancy_rate_prev_year as vacancy_rate_change
+
+    from with_yoy
+)
+
+select * from final