fix(backend): harden write registrar boundaries

backend/etc/agent.dev.yaml

@@ -2,6 +2,7 @@ env: development
 port: "9150"
 dsn: "postgres://postgres:postgres@localhost:5432/agent_dev?sslmode=disable"
 storage_dir: "./data/files"
+max_upload_bytes: 33554432
 auth_secret: "development-auth-secret-change-me"
 system_ai_key: ""
 ai_key_encryption_secret: "development-ai-key-secret-change-me"