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/app/api/v1/router.py

@@ -0,0 +1,9 @@
+from fastapi import APIRouter
+
+from app.api.v1.endpoints import accounts, auth, categories, transactions
+
+api_router = APIRouter(prefix="/api/v1")
+api_router.include_router(auth.router)
+api_router.include_router(accounts.router)
+api_router.include_router(categories.router)
+api_router.include_router(transactions.router)