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/contact.py

@@ -142,6 +142,14 @@ class Contact(Base, TenantMixin):
         PGUUID(as_uuid=True), ForeignKey("contactpersons.id", ondelete="SET NULL"), nullable=True
     )
 
+    # ── Folder assignment ──
+    folder_id: Mapped[uuid.UUID | None] = mapped_column(
+        PGUUID(as_uuid=True),
+        ForeignKey("contact_folders.id", ondelete="SET NULL"),
+        nullable=True,
+        index=True,
+    )
+
     # ── Custom fields ──
     custom: Mapped[dict | None] = mapped_column(JSON, nullable=True, default=dict)