feat(akte): FILE-009 Freitext-Notizen an beliebiger Ressource

Polymorphes akte_notiz-Modell (gleiches entitaet_typ/entitaet_id-Muster wie
Dokument/Historie/Mangel), CRUD-Endpunkte, NotizenPanel im Akte-Grid. Löschen
nur durch Autor oder Administrator.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CVgbozhYmuEhiEJHffRXCV

backend/app/models/notiz.py

@@ -0,0 +1,22 @@
+import uuid
+from datetime import datetime
+
+from sqlalchemy import ForeignKey, String
+from sqlalchemy.dialects.postgresql import TIMESTAMP, UUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.db.base import Base
+
+
+class AkteNotiz(Base):
+    """FILE-009: formlose Freitext-Notiz an beliebiger Ressource - gleiches
+    entitaet_typ/entitaet_id-Muster wie Dokument/Historie/Mangel."""
+
+    __tablename__ = "akte_notiz"
+
+    id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    entitaet_typ: Mapped[str] = mapped_column(String, nullable=False)
+    entitaet_id: Mapped[str] = mapped_column(String, nullable=False)
+    text: Mapped[str] = mapped_column(String, nullable=False)
+    autor_id: Mapped[int] = mapped_column(ForeignKey("benutzer.id"), nullable=False)
+    erstellt_am: Mapped[datetime] = mapped_column(TIMESTAMP(timezone=True), nullable=False)