fix: PATCH-Buchung ohne Feld laesst Kategorie unveraendert, 404 bei unbekannter Kategorie

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

finance/app/routers/transactions.py

@@ -8,7 +8,7 @@ from sqlalchemy.orm import Session
 
 from app.auth import require_auth
 from app.db import get_session
-from app.models.tables import Transaction
+from app.models.tables import Category, Transaction
 from app.parsers.validate import dedup_hash
 from app.services.categorize import apply_rules
 
@@ -110,7 +110,10 @@ def patch_transaction(tx_id: int, data: TransactionPatch,
     tx = session.get(Transaction, tx_id)
     if tx is None:
         raise HTTPException(404, "Buchung nicht gefunden")
-    tx.category_id = data.category_id
+    if "category_id" in data.model_fields_set:
+        if data.category_id is not None and session.get(Category, data.category_id) is None:
+            raise HTTPException(404, "Kategorie nicht gefunden")
+        tx.category_id = data.category_id
     session.commit()
     session.refresh(tx)
     return TransactionOut.model_validate(tx)