appview: delete tag records with the encoded rkey on manifest delete

DeleteManifestHandler built the tag rkey as "repo:tag" while the write
path uses RepositoryTagToRKey, which is "repo_tag" with "/" encoded as
"~". For a nested repo like stream/cache the two never match, so the
cascade leaves the tag record on the PDS while removing the local cache
row, and the tag reappears on the next backfill. Depending on the
variant it either no-ops (deleteRecord is idempotent) or fails outright
on an rkey containing a slash.

Every other io.atcr.tag call site already routes through the helper.
This was the last hand-built one.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

pkg/appview/handlers/images.go

@@ -178,8 +178,11 @@ func (h *DeleteManifestHandler) ServeHTTP(w http.ResponseWriter, r *http.Request
 
 		// Delete each tag from PDS and database
 		for _, tag := range tags {
-			// Delete from PDS
-			tagRKey := fmt.Sprintf("%s:%s", repo, tag)
+			// Delete from PDS. Must go through RepositoryTagToRKey: the write
+			// path encodes repository slashes as "~", so hand-building the
+			// rkey here targets a record that doesn't exist (a silent no-op,
+			// since deleteRecord is idempotent) for any repo with a "/" in it.
+			tagRKey := atproto.RepositoryTagToRKey(repo, tag)
 			if err := pdsClient.DeleteRecord(r.Context(), atproto.TagCollection, tagRKey); err != nil {
 				// Check if OAuth error - if so, invalidate sessions and return 401
 				if handleOAuthError(r.Context(), h.Refresher, user.DID, err) {