feat: Kontakt-Ordner mit Drag&Drop Baum (wie KI-Chat Sidebar)

- Backend: ContactFolder model (hierarchisch, parent_id, sort_order)
- Migration 0022: contact_folders Tabelle + folder_id FK auf contacts
- CRUD Routes: /api/v1/contact-folders (list, create, update, delete, reorder)
- Move Contact API: /api/v1/contact-folders/contacts/{id}/move
- folder_id Filter in contacts list API
- Frontend: ContactFolderTree mit Baum-Struktur, Drag&Drop, Kontext-Menü
- Kontakt-Personen-Icon statt Ordner-Symbol
- ContactList Items draggable (List/Table/Cards View)
- React Query Hooks für Folder CRUD + Move Contact
- Alle 266 Frontend-Tests passing

app/models/__init__.py

@@ -7,6 +7,7 @@ from app.models.audit import AuditLog, DeletionLog
 from app.models.auth import ApiToken, PasswordResetToken
 from app.models.company import Company
 from app.models.contact import Contact, ContactPerson
+from app.models.contact_folder import ContactFolder
 from app.models.currency import Currency
 from app.models.group import Group, UserGroup
 from app.models.notification import Notification, NotificationPreference, NotificationType
@@ -38,6 +39,7 @@ __all__ = [
     "Company",
     "Contact",
     "ContactPerson",
+    "ContactFolder",
     "Currency",
     "TaxRate",
     "Sequence",