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/auth.py

@@ -0,0 +1,27 @@
+from datetime import datetime, timedelta
+
+from fastapi import Depends, HTTPException, status
+from fastapi.security import OAuth2PasswordBearer
+from jose import JWTError, jwt
+
+from app.config import settings
+
+oauth2_scheme = OAuth2PasswordBearer(tokenUrl="/api/v1/auth/login")
+
+ALGORITHM = "HS256"
+
+
+def create_access_token(subject: str) -> str:
+    expire = datetime.utcnow() + timedelta(minutes=settings.ACCESS_TOKEN_EXPIRE_MINUTES)
+    return jwt.encode({"sub": subject, "exp": expire}, settings.SECRET_KEY, algorithm=ALGORITHM)
+
+
+def get_current_user(token: str = Depends(oauth2_scheme)) -> str:
+    try:
+        payload = jwt.decode(token, settings.SECRET_KEY, algorithms=[ALGORITHM])
+        username: str = payload.get("sub")
+        if username is None:
+            raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED)
+        return username
+    except JWTError:
+        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED)