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>

portfolio_app/toronto/loaders/base.py

@@ -0,0 +1,85 @@
+"""Base loader utilities for database operations."""
+
+from collections.abc import Generator
+from contextlib import contextmanager
+from typing import Any, TypeVar
+
+from sqlalchemy.orm import Session
+
+from portfolio_app.toronto.models import get_session_factory
+
+T = TypeVar("T")
+
+
+@contextmanager
+def get_session() -> Generator[Session, None, None]:
+    """Get a database session with automatic cleanup.
+
+    Yields:
+        SQLAlchemy session that auto-commits on success, rollbacks on error.
+    """
+    session_factory = get_session_factory()
+    session = session_factory()
+    try:
+        yield session
+        session.commit()
+    except Exception:
+        session.rollback()
+        raise
+    finally:
+        session.close()
+
+
+def bulk_insert(session: Session, objects: list[T]) -> int:
+    """Bulk insert objects into the database.
+
+    Args:
+        session: Active SQLAlchemy session.
+        objects: List of ORM model instances to insert.
+
+    Returns:
+        Number of objects inserted.
+    """
+    session.add_all(objects)
+    session.flush()
+    return len(objects)
+
+
+def upsert_by_key(
+    session: Session,
+    model_class: Any,
+    objects: list[T],
+    key_columns: list[str],
+) -> tuple[int, int]:
+    """Upsert objects based on unique key columns.
+
+    Args:
+        session: Active SQLAlchemy session.
+        model_class: The ORM model class.
+        objects: List of ORM model instances to upsert.
+        key_columns: Column names that form the unique key.
+
+    Returns:
+        Tuple of (inserted_count, updated_count).
+    """
+    inserted = 0
+    updated = 0
+
+    for obj in objects:
+        # Build filter for existing record
+        filters = {col: getattr(obj, col) for col in key_columns}
+        existing = session.query(model_class).filter_by(**filters).first()
+
+        if existing:
+            # Update existing record
+            for column in model_class.__table__.columns:
+                if column.name not in key_columns and column.name != "id":
+                    setattr(existing, column.name, getattr(obj, column.name))
+            updated += 1
+        else:
+            # Insert new record
+            session.add(obj)
+            inserted += 1
+
+    session.flush()
+    return inserted, updated