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_folder.py

@@ -0,0 +1,33 @@
+"""Contact folder schemas — CRUD for hierarchical contact folders."""
+
+from __future__ import annotations
+
+from pydantic import BaseModel, Field
+
+
+class ContactFolderCreate(BaseModel):
+    name: str = Field(..., min_length=1, max_length=255)
+    parent_id: str | None = None
+
+
+class ContactFolderUpdate(BaseModel):
+    name: str | None = Field(None, min_length=1, max_length=255)
+    parent_id: str | None = None
+    sort_order: int | None = None
+
+
+class ContactFolderResponse(BaseModel):
+    id: str
+    name: str
+    parent_id: str | None = None
+    user_id: str
+    sort_order: int = 0
+
+
+class ContactFolderTreeResponse(ContactFolderResponse):
+    children: list["ContactFolderTreeResponse"] = Field(default_factory=list)
+    contact_count: int = 0
+
+
+class MoveContactRequest(BaseModel):
+    folder_id: str | None = None