Add local gems repositories (#153)

In-house gems have nowhere to publish: gems locals only take raw file uploads, which `gem push`, `gem install` and bundler cannot use.

- accept `gem push` (`POST <repo>/api/v1/gems`, 256MiB cap, 8MiB metadata cap) and `gem yank` (`DELETE <repo>/api/v1/gems/yank`); any API key works
- parse the gemspec from the `.gem`; reject an existing name-version-platform with 409
- serve the compact index (`versions`, `info/<gem>`, `names`), `.gem` files and generated `quick/Marshal.4.8` specs; no legacy `specs.4.8.gz`
- route HEAD on locals (gem probes `versions` with HEAD)
- add `docs/gems/local.md`, a UI snippet and a stock `ruby:3.3` e2e-docker case

Reviewed-on: #153
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

docs/gems/local.md

@@ -0,0 +1,24 @@
+# Local gems repository
+
+A local `gems` repo is a RubyGems source: `gem push`, `gem yank`, `gem install`
+and `bundle install` work against `/api/v1/local/<repo>`. `gem push` needs an
+API key; any value works.
+
+```bash
+GEM_HOST_API_KEY=unused gem push --host https://artifactapi.example.com/api/v1/local/<repo> mygem-1.0.0.gem
+gem install mygem --source https://artifactapi.example.com/api/v1/local/<repo>/
+gem yank mygem -v 1.0.0 --host https://artifactapi.example.com/api/v1/local/<repo>
+```
+
+```ruby
+# Gemfile
+source "https://artifactapi.example.com/api/v1/local/<repo>/"
+gem "mygem"
+```
+
+The repo serves the compact index (`versions`, `info/<gem>`, `names`), `gems/*.gem`
+and `quick/Marshal.4.8/*.gemspec.rz`. The legacy `specs.4.8.gz` indexes are not
+served; current gem and bundler clients use the compact index.
+
+Versions are immutable: pushing an existing name-version-platform is rejected
+with 409. A yanked version disappears from the index and its files are removed.