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>

.env.example

@@ -0,0 +1,17 @@
+# ── PostgreSQL ──────────────────────────────────────────
+POSTGRES_USER=budget
+POSTGRES_PASSWORD=budget
+POSTGRES_DB=budget_tracker
+DB_PORT=5432
+
+# ── Backend (FastAPI) ──────────────────────────────────
+DATABASE_URL=postgresql+asyncpg://budget:budget@db:5432/budget_tracker
+SECRET_KEY=change-me-in-production
+ACCESS_TOKEN_EXPIRE_MINUTES=15
+REFRESH_TOKEN_EXPIRE_DAYS=7
+CORS_ORIGINS=["http://localhost:5173","http://localhost:3000"]
+DEBUG=false
+BACKEND_PORT=8000
+
+# ── Frontend ───────────────────────────────────────────
+FRONTEND_PORT=3000