Add local npm registry publish and install (#139)

In-house npm packages have nowhere to publish: npm locals only accept raw file uploads, which `npm publish` and `npm install` cannot use.

- accept `npm publish` (`PUT /api/v1/local/<repo>/<pkg>`, scoped included) on local npm repos, capping the body at 256MiB and verifying the tarball against dist integrity
- write tarball, then manifest, then dist-tags; publishing an existing version returns 409 and leaves tags alone, while a matching leftover tarball from a failed publish is reused so retries complete
- serve packuments with tarball URLs pointing back at the local repo, dropping dist-tags that name unpublished versions
- document the `.npmrc` registry + `_authToken` setup in the UI and `docs/npm/local.md`
- add a stock `node:22` publish/install e2e-docker case

Reviewed-on: #139
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

docs/npm/local.md

@@ -0,0 +1,14 @@
+# Local npm registry
+
+A local `npm` repo is a real npm registry: `npm publish` (scoped and unscoped)
+and `npm install` work against `/api/v1/local/<repo>/`. npm requires an
+`_authToken` for the registry before it will publish; any value works.
+
+```ini
+# .npmrc
+registry=https://artifactapi.example.com/api/v1/local/<repo>/
+//artifactapi.example.com/api/v1/local/<repo>/:_authToken=unused
+```
+
+Versions are immutable: republishing an existing version is rejected. The
+dist-tags sent with a publish (`latest`, or `--tag`) move to that version.