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>

migrations/0005_helm_metadata.sql

@@ -0,0 +1,13 @@
+CREATE TABLE IF NOT EXISTS helm_metadata (
+	id           BIGSERIAL PRIMARY KEY,
+	repo_name    TEXT NOT NULL,
+	file_path    TEXT NOT NULL,
+	content_hash TEXT NOT NULL,
+	name         TEXT NOT NULL,
+	version      TEXT NOT NULL,
+	chart_yaml   TEXT NOT NULL,
+	created_at   TIMESTAMPTZ DEFAULT NOW(),
+	UNIQUE(repo_name, file_path)
+);
+
+CREATE INDEX IF NOT EXISTS idx_helm_metadata_repo ON helm_metadata(repo_name);