feat: add takeover fields to Authorization model

Add takeover_enabled, takeover_mode, and takeover_delay_seconds columns
to the Authorization model with a SQLite migration, and include them
in to_dict(). Test: test_takeover_model.py.

digital-avatar-app/backend/database.py

@@ -35,6 +35,9 @@ def init_db():
         ("knowledge_docs", "chunk_count", "INTEGER DEFAULT 0"),
         ("knowledge_docs", "vectorized_at", "TIMESTAMP"),
         ("avatars", "owner_id", "VARCHAR DEFAULT ''"),
+        ("authorizations", "takeover_enabled", "BOOLEAN DEFAULT 0"),
+        ("authorizations", "takeover_mode", "VARCHAR DEFAULT 'immediate'"),
+        ("authorizations", "takeover_delay_seconds", "INTEGER DEFAULT 30"),
     )