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>

scripts/db/init-postgis.sql

@@ -0,0 +1,8 @@
+-- Initialize PostGIS extension
+-- This script runs automatically on first container start
+
+-- Enable PostGIS extension
+CREATE EXTENSION IF NOT EXISTS postgis;
+
+-- Verify installation
+SELECT PostGIS_Version();