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/dbt_project.yml

@@ -1,8 +1,7 @@
-name: 'toronto_housing'
-version: '1.0.0'
+name: 'portfolio'
 config-version: 2
 
-profile: 'toronto_housing'
+profile: 'portfolio'
 
 model-paths: ["models"]
 analysis-paths: ["analyses"]
@@ -16,13 +15,19 @@ clean-targets:
   - "dbt_packages"
 
 models:
-  toronto_housing:
+  portfolio:
+    shared:
+      +materialized: view
+      +schema: shared
     staging:
-      +materialized: view
-      +schema: staging
+      toronto:
+        +materialized: view
+        +schema: staging
     intermediate:
-      +materialized: view
-      +schema: intermediate
+      toronto:
+        +materialized: view
+        +schema: intermediate
     marts:
-      +materialized: table
-      +schema: marts
+      toronto:
+        +materialized: table
+        +schema: marts