Declare proprietary licensing

Continuum is intended to be sold, so every dependency has to permit
distribution in closed-source commercial software. This package carried
no licence field; `"UNLICENSED"` with `"private": true` is npm's
convention for that. The policy and the dependency audit are in
continuum-common's docs/LICENSES.md.

No `deny.toml` here - there are no cargo dependencies in this repo.

package.json

@@ -1,5 +1,6 @@
 {
   "name": "@continuum/backend",
+  "license": "UNLICENSED",
   "version": "0.1.0",
   "private": true,
   "type": "module",