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/alembic/versions/0035_akte_notiz.py

@@ -0,0 +1,34 @@
+"""FILE-009: Freitext-Notizen in der Akte.
+
+Revision ID: 0035_akte_notiz
+Revises: 0034_person
+Create Date: 2026-09-10
+"""
+from typing import Sequence, Union
+
+from alembic import op
+
+revision: str = "0035_akte_notiz"
+down_revision: Union[str, None] = "0034_person"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.execute(
+        """
+        CREATE TABLE akte_notiz (
+            id UUID PRIMARY KEY,
+            entitaet_typ TEXT NOT NULL,
+            entitaet_id TEXT NOT NULL,
+            text TEXT NOT NULL,
+            autor_id INTEGER NOT NULL REFERENCES benutzer(id),
+            erstellt_am TIMESTAMPTZ NOT NULL
+        )
+        """
+    )
+    op.execute("CREATE INDEX ix_akte_notiz_entitaet ON akte_notiz (entitaet_typ, entitaet_id)")
+
+
+def downgrade() -> None:
+    op.execute("DROP TABLE akte_notiz")