Add helm local repositories (#140)

In-house charts have nowhere to publish: a helm local stores uploads but serves no index.yaml, and the helm virtual merger drops every chart field except name/version/urls.

- add helm local uploads (`PUT .../files/<name>-<version>.tgz`) recording each Chart.yaml in a new `helm_metadata` table; mismatched filenames stay out of the index
- bound chart parsing (50MiB decompressed, 10k entries, 1MiB Chart.yaml)
- generate a deterministic `index.yaml` with sha256 digests and path-escaped relative urls
- keep all chart fields when merging helm virtual indexes, expanding YAML aliases
- add a real-helm e2e-docker test, UI snippet and README section

Reviewed-on: #140
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

ui/src/components/UsageInstructions.tsx

@@ -167,12 +167,23 @@ docker push        ${host}/${name}/myapp:latest`,
     case 'helm':
       return [
         {
-          title: 'Add the Helm repo',
+          title: isLocal ? 'Add the Helm repo (index.yaml auto-generated)' : 'Add the Helm repo',
           language: 'bash',
-          code: `helm repo add ${name} ${proxy}/
+          code: `helm repo add ${name} ${isLocal ? `${url}/api/v1/local/${name}` : proxy}/
 helm repo update
 helm install <release> ${name}/<chart>`,
         },
+        ...(isLocal
+          ? [
+              {
+                title: 'Publish a chart',
+                language: 'bash',
+                code: `helm package ./mychart
+curl -fsSL --upload-file mychart-0.1.0.tgz \\
+  ${url}/api/v2/remotes/${name}/files/mychart-0.1.0.tgz`,
+              },
+            ]
+          : []),
       ];
 
     case 'alpine':