Add budget module: FastAPI backend + React frontend

Backend: FastAPI + PostgreSQL with models for accounts, transactions,
and categories. Auto-categorization from merchant patterns, token auth,
CRUD endpoints, and seed data for 16 categories and 4 bank accounts.

Frontend: Login, Dashboard (account balances + recent charges),
Transactions (full CRUD table with search/filter), Cash & Transfers
view. Dark theme with emerald/cyan accents, responsive layout.

Infrastructure: Updated docker-compose for backend + db services,
nginx proxy config for API routing, deploy workflow with secrets.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/requirements.txt

@@ -0,0 +1,9 @@
+fastapi[standard]
+uvicorn[standard]
+sqlmodel
+psycopg2-binary
+python-jose[cryptography]
+passlib[bcrypt]
+python-multipart
+python-dotenv
+alembic