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/lib/subscriptionCache.test.ts

@@ -0,0 +1,38 @@
+import { readdirSync, readFileSync } from "node:fs";
+import { QueryClient } from "@tanstack/react-query";
+import { describe, expect, it, vi } from "vitest";
+
+import { qk } from "./queryKeys";
+import { invalidateSubscription } from "./subscriptionCache";
+
+// E28 review L8: a subscription change touches the same reads wherever it is made, and each site kept
+// its own list — the channel menu's unsubscribe left the feed showing the channel's videos, the
+// channel page's left the status counts stale. One set, one function, every site on it.
+describe("a subscription change", () => {
+  it("refreshes the channel, the lists, the counts and the feed", () => {
+    const qc = new QueryClient();
+    const spy = vi.spyOn(qc, "invalidateQueries");
+    invalidateSubscription(qc, "UC1");
+    expect(spy.mock.calls.map(([f]) => f?.queryKey)).toEqual([
+      qk.channel("UC1"),
+      qk.channels(),
+      qk.myStatus(),
+      qk.feed(),
+      qk.feedSummary(),
+      qk.discoveredChannels(),
+    ]);
+  });
+
+  it("is refreshed through that one function wherever it is made", () => {
+    const root = new URL("../components/", import.meta.url);
+    const sites = readdirSync(root)
+      .filter((f) => f.endsWith(".tsx") && !f.includes(".test."))
+      .map((f) => [f, readFileSync(new URL(f, root), "utf8")] as const)
+      .filter(([, src]) => /api\.(un)?subscribeChannel\(/.test(src));
+    expect(sites.length).toBeGreaterThanOrEqual(3);
+    for (const [f, src] of sites) {
+      const calls = src.match(/api\.(un)?subscribeChannel\(/g)!.length;
+      expect(src.match(/invalidateSubscription\(/g)?.length ?? 0, f).toBe(calls);
+    }
+  });
+});