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/schemas/contact.py

@@ -141,6 +141,8 @@ class ContactCreate(BaseModel):
     image: str | None = None
     # Custom
     custom: dict | None = None
+    # Folder assignment
+    folder_id: str | None = None
     # Contact persons (optional inline create)
     contact_persons: list[ContactPersonCreate] | None = None
 
@@ -208,6 +210,7 @@ class ContactUpdate(BaseModel):
     tags: str | None = Field(None, max_length=500)
     image: str | None = None
     custom: dict | None = None
+    folder_id: str | None = None
     default_person_id: str | None = None
     admin_contactperson_id: str | None = None
 
@@ -278,6 +281,7 @@ class ContactResponse(BaseModel):
     tags: str | None = None
     image: str | None = None
     custom: dict | None = None
+    folder_id: str | None = None
     default_person_id: str | None = None
     admin_contactperson_id: str | None = None
     created_at: str | None = None