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/shared/stg_dimensions__time.sql

@@ -0,0 +1,22 @@
+-- Staged time dimension
+-- Source: shared.dim_time table
+-- Grain: One row per month
+-- Note: Shared dimension used across all dashboard projects
+
+with source as (
+    select * from {{ source('shared', 'dim_time') }}
+),
+
+staged as (
+    select
+        date_key,
+        full_date,
+        year,
+        month,
+        quarter,
+        month_name,
+        is_month_start
+    from source
+)
+
+select * from staged