feat: Konto-Saldo-Anker mit ankerbasierter Saldenrechnung

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

finance/app/routers/accounts.py

@@ -1,3 +1,4 @@
+from datetime import date
 from decimal import Decimal
 
 from fastapi import APIRouter, Depends, HTTPException
@@ -25,14 +26,23 @@ class AccountOut(AccountIn):
     model_config = ConfigDict(from_attributes=True)
     id: int
     balance: Decimal = Decimal("0")
+    anchor_date: date | None = None
+    anchor_balance: Decimal | None = None
 
 
 class AccountPatch(BaseModel):
-    name: str
+    # Alle Felder optional; model_fields_set entscheidet, was tatsaechlich
+    # geaendert wird (Muster wie TransactionPatch.category_id). anchor_date
+    # und anchor_balance muessen gemeinsam gesetzt oder gemeinsam null sein.
+    name: str | None = None
+    anchor_date: date | None = None
+    anchor_balance: Decimal | None = None
 
     @field_validator("name")
     @classmethod
-    def name_not_blank(cls, v: str) -> str:
+    def name_not_blank(cls, v: str | None) -> str | None:
+        if v is None:
+            return v
         v = v.strip()
         if not v or len(v) > 100:
             raise ValueError("Name muss 1–100 Zeichen lang sein")
@@ -75,7 +85,25 @@ def patch_account(account_id: int, data: AccountPatch, session: Session = Depend
     acc = session.get(Account, account_id)
     if acc is None:
         raise HTTPException(404, "Konto nicht gefunden")
-    acc.name = data.name
+
+    # Validierung zuerst, VOR jeder Mutation - sonst haengt bei einem 422 auf
+    # dem Anker eine bereits geschriebene name-Aenderung im Session-State.
+    fields = data.model_fields_set
+    if "name" in fields and data.name is None:
+        raise HTTPException(422, "Name darf nicht leer sein")
+
+    anchor_fields = {"anchor_date", "anchor_balance"} & fields
+    if anchor_fields and (
+            anchor_fields != {"anchor_date", "anchor_balance"}
+            or (data.anchor_date is None) != (data.anchor_balance is None)):
+        raise HTTPException(422, "Anker braucht Datum und Betrag (oder beide leeren)")
+
+    if "name" in fields:
+        acc.name = data.name
+    if anchor_fields:
+        acc.anchor_date = data.anchor_date
+        acc.anchor_balance = data.anchor_balance
+
     session.commit()
     session.refresh(acc)
     item = AccountOut.model_validate(acc)