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/endpoints/auth.py

@@ -0,0 +1,22 @@
+from fastapi import APIRouter, HTTPException, status
+from fastapi.security import OAuth2PasswordRequestForm
+from fastapi import Depends
+
+from app.auth import create_access_token
+from app.config import settings
+
+router = APIRouter(prefix="/auth", tags=["auth"])
+
+
+@router.post("/login")
+def login(form_data: OAuth2PasswordRequestForm = Depends()):
+    if (
+        form_data.username != settings.ADMIN_USERNAME
+        or form_data.password != settings.ADMIN_PASSWORD
+    ):
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail="Invalid credentials",
+        )
+    token = create_access_token(form_data.username)
+    return {"access_token": token, "token_type": "bearer"}