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/ChannelPage.tsx

@@ -4,6 +4,7 @@ import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { useLiveQuery } from "../lib/useLiveQuery";
 import { channelDetailPollMs, useBackfillFeedRefresh } from "../lib/channelDetailPoll";
 import { qk } from "../lib/queryKeys";
+import { invalidateSubscription } from "../lib/subscriptionCache";
 import { useNavigationActions } from "./navigationContext";
 import { PriorityStepper } from "./ui/PriorityStepper";
 import { useChannelPriority } from "../lib/useChannelPriority";
@@ -181,12 +182,7 @@ export default function ChannelPage({
   const subscribe = useMutation({
     mutationFn: () => api.subscribeChannel(channelId),
     onSuccess: () => {
-      qc.invalidateQueries({ queryKey: qk.channel(channelId) });
-      qc.invalidateQueries({ queryKey: qk.feed() });
-      qc.invalidateQueries({ queryKey: qk.channels() });
-      // Match ChannelDiscovery: the channel leaves discovery and the manager's stats move.
-      qc.invalidateQueries({ queryKey: qk.myStatus() });
-      qc.invalidateQueries({ queryKey: qk.discoveredChannels() });
+      invalidateSubscription(qc, channelId);
       notify({ level: "info", message: t("channel.subscribed", { name: ch?.title ?? "" }) });
     },
     // A 403 (missing write scope) otherwise fails silently on the channel page (no wizard handle
@@ -195,11 +191,7 @@ export default function ChannelPage({
   });
   const unsubscribe = useMutation({
     mutationFn: () => api.unsubscribeChannel(channelId),
-    onSuccess: () => {
-      qc.invalidateQueries({ queryKey: qk.channel(channelId) });
-      qc.invalidateQueries({ queryKey: qk.feed() });
-      qc.invalidateQueries({ queryKey: qk.channels() });
-    },
+    onSuccess: () => invalidateSubscription(qc, channelId),
     onError: (err) => notifyYouTubeActionError(err, t("channels.notify.unsubscribeFailed")),
   });
   // "Get full history": the catalog only holds the recent-backfill/RSS subset of a channel's