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/api/v2/objects.go

@@ -75,7 +75,7 @@ func (h *ObjectsHandler) evictLocal(w http.ResponseWriter, r *http.Request) {
 	repoName := chi.URLParam(r, "name")
 	path := chi.URLParam(r, "*")
 
-	if err := h.db.DeleteLocalFile(r.Context(), repoName, path); err != nil {
+	if err := deleteLocalFile(r.Context(), h.db, repoName, path); err != nil {
 		http.Error(w, fmt.Sprintf("evict failed: %v", err), http.StatusInternalServerError)
 		return
 	}