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/local.go

@@ -185,13 +185,35 @@ func (h *LocalHandler) remove(w http.ResponseWriter, r *http.Request) {
 	repoName := chi.URLParam(r, "name")
 	filePath := chi.URLParam(r, "*")
 
-	if err := h.db.DeleteLocalFile(r.Context(), repoName, filePath); err != nil {
+	if err := deleteLocalFile(r.Context(), h.db, repoName, filePath); err != nil {
 		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return
 	}
 	w.WriteHeader(http.StatusNoContent)
 }
 
+// deleteLocalFile removes a local file and runs the provider's post-delete hook,
+// so provider-derived state (e.g. RPM metadata that feeds generated repodata)
+// stops referencing a package that no longer exists.
+func deleteLocalFile(ctx context.Context, db *database.DB, repoName, filePath string) error {
+	if err := db.DeleteLocalFile(ctx, repoName, filePath); err != nil {
+		return err
+	}
+
+	remote, err := db.GetRemote(ctx, repoName)
+	if err != nil {
+		return nil // file is gone; no repo left to resolve a cleanup hook from
+	}
+	prov, err := provider.Get(remote.PackageType)
+	if err != nil {
+		return nil
+	}
+	if hook, ok := prov.(provider.PostDeleteHook); ok {
+		return hook.AfterDelete(ctx, repoName, filePath, db)
+	}
+	return nil
+}
+
 func (h *LocalHandler) DB() *database.DB {
 	return h.db
 }