feat: Phase 1 – Konten, Kategorien, Transaktionen, EZB-Währungsumrechnung

Backend:
- Modelle + Migration 0002: accounts, account_identifiers, category_groups,
  categories, transactions, exchange_rates (alle owner-gescoped).
- EZB-Kursdienst (90-Tage/Historie) + Backfill-Skript; Umrechnung EUR<->CHF zum
  Buchungsdatum mit lokalem Cache und manueller Übersteuerung; EUR-Pivot
  (amount_eur_cents) eingefroren.
- Kontostände + Netto-Vermögen (Umrechnung in Hauptwährung).
- Router: accounts (inkl. IBAN-Identifikatoren), categories (Gruppen+Kategorien),
  transactions (Ausgabe/Einnahme/Transfer, cross-currency), rates.
- Gruppierte Standard-Kategorien je Nutzer beim Anlegen.
- pytest-Suite: Währung, Salden, Cross-Currency-Transfer, Nutzer-Isolation.

Frontend:
- Seiten Konten, Kategorien, Buchungen; Icon-Chip, Geldbetrag mit Umrechnung,
  Icon-/Farbwähler, Modal; erweitertes SVG-Icon-Set (Konto-/Kategorie-Icons).
- Währungs-/Datumsformatierung (EUR de-DE, CHF de-CH), i18n DE/EN erweitert.
- Live-Umrechnungs-Vorschau im Transfer-Formular.

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

backend/app/routers/categories.py

@@ -0,0 +1,154 @@
+"""Kategorie-Gruppen und Kategorien (zweistufig). Alles owner-gescoped.
+
+Eine Kategorie erbt ihre Art (Einnahme/Ausgabe) von ihrer Gruppe.
+"""
+
+from fastapi import APIRouter, Depends, status
+from sqlalchemy.orm import Session
+
+from ..database import get_db
+from ..deps import get_current_user, get_owned
+from ..models import Category, CategoryGroup, User
+from ..schemas import (
+    CategoryCreate,
+    CategoryGroupCreate,
+    CategoryGroupOut,
+    CategoryGroupUpdate,
+    CategoryOut,
+    CategoryUpdate,
+)
+
+router = APIRouter(prefix="/categories", tags=["categories"])
+
+
+def group_out(g: CategoryGroup) -> CategoryGroupOut:
+    cats = sorted(g.categories, key=lambda c: (c.position, c.name.lower()))
+    return CategoryGroupOut(
+        id=g.id,
+        name=g.name,
+        kind=g.kind,
+        icon=g.icon,
+        color=g.color,
+        position=g.position,
+        is_builtin=g.is_builtin,
+        categories=[CategoryOut.model_validate(c) for c in cats],
+    )
+
+
+def _next_position(db: Session, model, **filters) -> int:
+    q = db.query(model.position)
+    for k, v in filters.items():
+        q = q.filter(getattr(model, k) == v)
+    row = q.order_by(model.position.desc()).first()
+    return (row[0] + 1) if row else 0
+
+
+@router.get("", response_model=list[CategoryGroupOut])
+def list_categories(db: Session = Depends(get_db), user: User = Depends(get_current_user)):
+    groups = (
+        db.query(CategoryGroup)
+        .filter(CategoryGroup.owner_id == user.id)
+        .order_by(CategoryGroup.position, CategoryGroup.name)
+        .all()
+    )
+    return [group_out(g) for g in groups]
+
+
+# --- Gruppen ---------------------------------------------------------------
+
+
+@router.post("/groups", response_model=CategoryGroupOut, status_code=status.HTTP_201_CREATED)
+def create_group(
+    payload: CategoryGroupCreate, db: Session = Depends(get_db), user: User = Depends(get_current_user)
+):
+    g = CategoryGroup(
+        owner_id=user.id,
+        name=payload.name.strip(),
+        kind=payload.kind,
+        icon=payload.icon,
+        color=payload.color,
+        position=_next_position(db, CategoryGroup, owner_id=user.id),
+    )
+    db.add(g)
+    db.commit()
+    db.refresh(g)
+    return group_out(g)
+
+
+@router.patch("/groups/{group_id}", response_model=CategoryGroupOut)
+def update_group(
+    group_id: int,
+    payload: CategoryGroupUpdate,
+    db: Session = Depends(get_db),
+    user: User = Depends(get_current_user),
+):
+    g = get_owned(db, CategoryGroup, group_id, user)
+    for key, value in payload.model_dump(exclude_unset=True).items():
+        if value is not None:
+            setattr(g, key, value)
+    db.commit()
+    db.refresh(g)
+    return group_out(g)
+
+
+@router.delete("/groups/{group_id}", status_code=status.HTTP_204_NO_CONTENT)
+def delete_group(
+    group_id: int, db: Session = Depends(get_db), user: User = Depends(get_current_user)
+):
+    g = get_owned(db, CategoryGroup, group_id, user)
+    db.delete(g)
+    db.commit()
+
+
+# --- Kategorien ------------------------------------------------------------
+
+
+@router.post("", response_model=CategoryOut, status_code=status.HTTP_201_CREATED)
+def create_category(
+    payload: CategoryCreate, db: Session = Depends(get_db), user: User = Depends(get_current_user)
+):
+    group = get_owned(db, CategoryGroup, payload.group_id, user)
+    cat = Category(
+        owner_id=user.id,
+        group_id=group.id,
+        name=payload.name.strip(),
+        type=group.kind,  # erbt die Art der Gruppe
+        icon=payload.icon,
+        color=payload.color,
+        position=_next_position(db, Category, owner_id=user.id, group_id=group.id),
+    )
+    db.add(cat)
+    db.commit()
+    db.refresh(cat)
+    return CategoryOut.model_validate(cat)
+
+
+@router.patch("/{category_id}", response_model=CategoryOut)
+def update_category(
+    category_id: int,
+    payload: CategoryUpdate,
+    db: Session = Depends(get_db),
+    user: User = Depends(get_current_user),
+):
+    cat = get_owned(db, Category, category_id, user)
+    data = payload.model_dump(exclude_unset=True)
+    new_group_id = data.pop("group_id", None)
+    if new_group_id is not None:
+        group = get_owned(db, CategoryGroup, new_group_id, user)
+        cat.group_id = group.id
+        cat.type = group.kind  # Art an die neue Gruppe angleichen
+    for key, value in data.items():
+        if value is not None or key == "is_archived":
+            setattr(cat, key, value)
+    db.commit()
+    db.refresh(cat)
+    return CategoryOut.model_validate(cat)
+
+
+@router.delete("/{category_id}", status_code=status.HTTP_204_NO_CONTENT)
+def delete_category(
+    category_id: int, db: Session = Depends(get_db), user: User = Depends(get_current_user)
+):
+    cat = get_owned(db, Category, category_id, user)
+    db.delete(cat)
+    db.commit()