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/ChannelDiscovery.tsx

@@ -2,6 +2,7 @@ import { useEffect } from "react";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { qk } from "../lib/queryKeys";
+import { invalidateSubscription } from "../lib/subscriptionCache";
 import { UserPlus } from "lucide-react";
 import { api, type DiscoveredChannel } from "../lib/api";
 import { accountKey, LS } from "../lib/storage";
@@ -58,9 +59,7 @@ export default function ChannelDiscovery({
     onSuccess: (_data, c) => {
       // The channel moves from "discovery" to "subscribed", and its videos will start
       // arriving — refresh both lists plus the per-user status.
-      qc.invalidateQueries({ queryKey: qk.discoveredChannels() });
-      qc.invalidateQueries({ queryKey: qk.channels() });
-      qc.invalidateQueries({ queryKey: qk.myStatus() });
+      invalidateSubscription(qc, c.id);
       const name = c.title ?? c.id;
       // Name the channel and ship a typed payload so the inbox can offer "open in the
       // Channel manager" / "open on YouTube" links — kept after reload, when live