feat: project bootstrap and structure

Sprint 1 initialization:
- Project directory structure (portfolio_app/, tests/, dbt/, data/, scripts/)
- CLAUDE.md with AI assistant context
- pyproject.toml with all dependencies
- docker-compose.yml for PostgreSQL 16 + PostGIS
- Makefile with standard targets
- Pre-commit configuration (ruff, mypy)
- Environment template (.env.example)
- Error handling foundation (PortfolioError hierarchy)
- Test configuration (conftest.py, pytest config)

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

.env.example

@@ -0,0 +1,15 @@
+# Database Configuration
+DATABASE_URL=postgresql://portfolio:portfolio_dev@localhost:5432/portfolio
+POSTGRES_USER=portfolio
+POSTGRES_PASSWORD=portfolio_dev
+POSTGRES_DB=portfolio
+
+# Application Settings
+DASH_DEBUG=true
+SECRET_KEY=change-me-in-production
+
+# Logging
+LOG_LEVEL=INFO
+
+# Optional: dbt profile (defaults to profiles.yml)
+# DBT_PROFILES_DIR=.