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_folder.py

@@ -0,0 +1,57 @@
+"""ContactFolder model — hierarchical folders for organizing contacts.
+
+Supports nested folders via parent_id, per-user ownership, and sort_order
+for manual ordering via drag & drop.
+"""
+
+from __future__ import annotations
+
+import uuid
+from datetime import datetime
+
+from sqlalchemy import DateTime, ForeignKey, Index, Integer, String, func
+from sqlalchemy.dialects.postgresql import UUID as PGUUID
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.core.db import Base, TenantMixin
+
+
+class ContactFolder(Base, TenantMixin):
+    """Hierarchical folder for organizing contacts.
+
+    Folders are tenant-scoped and user-owned. A folder with parent_id=NULL
+    is a root folder. sort_order controls display order within the same parent.
+    """
+
+    __tablename__ = "contact_folders"
+    __table_args__ = (
+        Index("ix_contact_folders_tenant_parent", "tenant_id", "parent_id"),
+        Index("ix_contact_folders_user", "user_id"),
+    )
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        PGUUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    name: Mapped[str] = mapped_column(String(255), nullable=False)
+    parent_id: Mapped[uuid.UUID | None] = mapped_column(
+        PGUUID(as_uuid=True),
+        ForeignKey("contact_folders.id", ondelete="CASCADE"),
+        nullable=True,
+    )
+    user_id: Mapped[uuid.UUID] = mapped_column(
+        PGUUID(as_uuid=True), ForeignKey("users.id", ondelete="CASCADE"), nullable=False
+    )
+    sort_order: Mapped[int] = mapped_column(Integer, nullable=False, default=0, server_default="0")
+
+    # ── Relationships ──
+    parent: Mapped[ContactFolder | None] = relationship(
+        "ContactFolder",
+        remote_side="ContactFolder.id",
+        back_populates="children",
+    )
+    children: Mapped[list[ContactFolder]] = relationship(
+        "ContactFolder",
+        back_populates="parent",
+        cascade="all, delete-orphan",
+        order_by="ContactFolder.sort_order",
+    )