feat: Transfer-Schlagwort mit festem Zielkonto (z. B. REVOLUT -> Konto Revolut)

- Transfer-Regel kann direkt ein Zielkonto tragen (target_account_id); ohne ->
  weiterhin Auto-Erkennung aus dem Buchungstext (Konto-Texte)
- Migration 0010; Endpunkt aktualisiert bestehendes Wort statt Konflikt
- Import bevorzugt das feste Zielkonto, sonst _guess_target_account
- Konten-Seite: Transfer-Erkennung mit Zielkonto-Auswahl + Anzeige am Chip
- Test: festes Zielkonto greift ohne Konto-Texte (75 grün)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/routers/categories.py

@@ -9,7 +9,7 @@ from sqlalchemy.orm import Session
 
 from ..database import get_db
 from ..deps import get_current_user, get_owned
-from ..models import CategorizationRule, Category, CategoryGroup, MatchField, RuleSource, User
+from ..models import Account, CategorizationRule, Category, CategoryGroup, MatchField, RuleSource, User
 from ..schemas import (
     CategoryCreate,
     CategoryGroupCreate,
@@ -353,7 +353,10 @@ def add_keyword(
 
 
 def _transfer_keyword_out(rule: CategorizationRule) -> TransferKeywordOut:
-    return TransferKeywordOut(id=rule.id, word=rule.pattern, amount_cents=rule.amount_cents)
+    return TransferKeywordOut(
+        id=rule.id, word=rule.pattern, amount_cents=rule.amount_cents,
+        target_account_id=rule.target_account_id,
+    )
 
 
 @router.get("/transfer-keywords", response_model=list[TransferKeywordOut])
@@ -378,15 +381,21 @@ def add_transfer_keyword(
     pattern = normalize_text(payload.word)[:255]
     if not pattern:
         raise HTTPException(status_code=status.HTTP_400_BAD_REQUEST, detail="Leeres Schlagwort")
+    target_id = None
+    if payload.target_account_id is not None:
+        target_id = get_owned(db, Account, payload.target_account_id, user).id
+    # Gleiches Wort existiert schon als Transfer-Regel -> aktualisieren (Betrag/Zielkonto).
     existing = _find_any_rule(db, user, pattern, "transfer")
     if existing is not None:
-        if existing.is_transfer and existing.amount_cents == payload.amount_cents:
-            return _transfer_keyword_out(existing)
-        raise _conflict(db, existing)
+        existing.amount_cents = payload.amount_cents
+        existing.target_account_id = target_id
+        db.commit()
+        db.refresh(existing)
+        return _transfer_keyword_out(existing)
     rule = CategorizationRule(
         owner_id=user.id, match_field=MatchField.any, pattern=pattern,
         category_id=None, amount_cents=payload.amount_cents, is_transfer=True,
-        kind="transfer", source=RuleSource.manual, priority=10,
+        target_account_id=target_id, kind="transfer", source=RuleSource.manual, priority=10,
     )
     db.add(rule)
     db.commit()