refactor: multi-dashboard structural migration

- Rename dbt project from toronto_housing to portfolio
- Restructure dbt models into domain subdirectories:
  - shared/ for cross-domain dimensions (dim_time)
  - staging/toronto/, intermediate/toronto/, marts/toronto/
- Update SQLAlchemy models for raw_toronto schema
- Add explicit cross-schema FK relationships for FactRentals
- Namespace figure factories under figures/toronto/
- Namespace notebooks under notebooks/toronto/
- Update Makefile with domain-specific targets and env loading
- Update all documentation for multi-dashboard structure

This enables adding new dashboard projects (e.g., /football, /energy)
without structural conflicts or naming collisions.

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

dbt/models/intermediate/toronto/int_year_spine.sql

@@ -0,0 +1,11 @@
+-- Intermediate: Year spine for analysis
+-- Creates a row for each year from 2014-2025
+-- Used to drive time-series analysis across all data sources
+
+with years as (
+    -- Generate years from available data sources
+    -- Crime data: 2014-2024, Rentals: 2019-2025
+    select generate_series(2014, 2025) as year
+)
+
+select year from years