chore: initial project setup

Phase 0 — full project scaffold with:
- Backend: FastAPI + SQLAlchemy 2.0 async + Alembic + PostgreSQL 16
- Frontend: React 18 + TypeScript + Vite + Tailwind CSS + shadcn/ui
- Docker Compose (prod + dev override with hot-reload)
- Health endpoint, CORS config, API proxy, env template

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/config.py

@@ -0,0 +1,17 @@
+from pydantic_settings import BaseSettings
+
+
+class Settings(BaseSettings):
+    """Application settings loaded from environment variables."""
+
+    DATABASE_URL: str = "postgresql+asyncpg://budget:budget@db:5432/budget_tracker"
+    SECRET_KEY: str = "change-me-in-production"
+    ACCESS_TOKEN_EXPIRE_MINUTES: int = 15
+    REFRESH_TOKEN_EXPIRE_DAYS: int = 7
+    CORS_ORIGINS: list[str] = ["http://localhost:5173"]
+    DEBUG: bool = False
+
+    model_config = {"env_file": ".env", "env_file_encoding": "utf-8"}
+
+
+settings = Settings()