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/db.py

@@ -0,0 +1,14 @@
+from sqlmodel import SQLModel, Session, create_engine
+
+from app.config import settings
+
+engine = create_engine(settings.DATABASE_URL)
+
+
+def init_db():
+    SQLModel.metadata.create_all(engine)
+
+
+def get_session():
+    with Session(engine) as session:
+        yield session