fix(channels): every subscribe and unsubscribe refreshes the same reads

Each of the four sites kept its own invalidation list and each missed something (the channel menu's
unsubscribe left the feed showing the channel's videos; the channel page's left the counts stale).
lib/subscriptionCache invalidateSubscription is the one set; a contract test holds every call site
to it.

frontend/src/components/useChannelActions.tsx

@@ -2,6 +2,7 @@ import { useMutation, useQueryClient } from "@tanstack/react-query";
 import { useTranslation } from "react-i18next";
 import { ArrowUpDown, EyeOff, ExternalLink, History, Tags, Tv, UserMinus } from "lucide-react";
 import { qk } from "../lib/queryKeys";
+import { invalidateSubscription } from "../lib/subscriptionCache";
 import { api, type ManagedChannel, type Tag } from "../lib/api";
 import { priorityChoices } from "../lib/channelOrder";
 import { channelYouTubeUrl } from "../lib/format";
@@ -79,8 +80,7 @@ export function useChannelActions({
   const unsubscribe = useMutation({
     mutationFn: (c: ManagedChannel) => api.unsubscribeChannel(c.id),
     onSuccess: (_d, c) => {
-      qc.invalidateQueries({ queryKey: qk.channels() });
-      qc.invalidateQueries({ queryKey: qk.myStatus() });
+      invalidateSubscription(qc, c.id);
       notify({ level: "success", message: t("channels.notify.unsubscribed", { name: nameOf(c) }) });
     },
     onError: (e) =>