fix: prune RPM metadata when a local file is evicted (#100)

Follow-up to #99.

## Why

Evicting or deleting a local RPM removed the \`local_files\` row but left its \`rpm_metadata\` behind. Since generated repodata is built from \`rpm_metadata\`, \`primary.xml\` kept advertising a package that no longer exists, producing 404s for clients that tried to fetch it.

## Changes

- Add \`PostDeleteHook\` and \`MetadataDeleter\` provider interfaces (symmetric to the existing \`PostUploadHook\`/\`MetadataStore\`), plus a \`DeleteRPMMetadata\` DB method.
- Implement \`AfterDelete\` in the RPM provider to drop the metadata row for the deleted file.
- Route both local delete paths — the new \`evictLocal\` and the existing files handler's \`remove\` — through a shared \`deleteLocalFile\` helper that removes the file then runs the provider's post-delete hook. Non-RPM providers have no hook, so nothing changes for them.
- Cover the cleanup with a dockerised test.

Reviewed-on: #100
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

internal/provider/provider.go

@@ -53,10 +53,20 @@ type PostUploadHook interface {
 	AfterUpload(ctx context.Context, repoName, storagePath, contentHash string, blobs BlobReader, db MetadataStore)
 }
 
+// PostDeleteHook lets a provider clean up derived state (e.g. RPM metadata that
+// feeds generated repodata) after a local file is removed.
+type PostDeleteHook interface {
+	AfterDelete(ctx context.Context, repoName, storagePath string, db MetadataDeleter) error
+}
+
 type MetadataStore interface {
 	InsertRPMMetadata(ctx context.Context, meta *RPMMetadata) error
 }
 
+type MetadataDeleter interface {
+	DeleteRPMMetadata(ctx context.Context, repoName, filePath string) error
+}
+
 type RPMMetadataReader interface {
 	ListRPMMetadataEntries(ctx context.Context, repoName string) ([]RPMMetadata, error)
 }