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/_shared.yml

@@ -0,0 +1,33 @@
+version: 2
+
+models:
+  - name: stg_dimensions__time
+    description: "Staged time dimension - shared across all projects"
+    columns:
+      - name: date_key
+        description: "Primary key (YYYYMM format)"
+        data_tests:
+          - unique
+          - not_null
+      - name: full_date
+        description: "First day of month"
+        data_tests:
+          - not_null
+      - name: year
+        description: "Calendar year"
+        data_tests:
+          - not_null
+      - name: month
+        description: "Month number (1-12)"
+        data_tests:
+          - not_null
+      - name: quarter
+        description: "Quarter (1-4)"
+        data_tests:
+          - not_null
+      - name: month_name
+        description: "Month name"
+        data_tests:
+          - not_null
+      - name: is_month_start
+        description: "Always true (monthly grain)"