refactor(api): honest FeedResponse; derive pageTitleKey from the registry (R7 S3a·2-3)

- FeedResponse claimed a required `limit`, but /api/search/youtube never
  returns it (only /api/feed does) and nothing in the app reads it. Drop the
  field so searchYoutube's Promise<FeedResponse> stops lying; documented for
  re-add if a consumer appears.
- pageMeta's hand-written 13-case Page->title switch now derives from the
  ModuleDef registry via a new optional `titleKey` (the longer tab title, e.g.
  "Channel manager"; falls back to the short nav labelKey). A new page is one
  touch (add it to MODULES) instead of also editing a switch. A test pins all
  13 title keys byte-identical to the old switch (suite 74).

frontend/src/lib/pageMeta.ts

@@ -1,36 +1,12 @@
 import type { Page } from "./urlState";
+import { moduleDef } from "./modules";
 
-// The i18n key for a page's human title — shared by the top-bar header and the browser tab title
-// so the two never drift. Keep in sync with the nav modules in NavSidebar / App's page switch.
+// The i18n key for a page's human title — shared by the top-bar header and the browser tab title so
+// the two never drift. Derived from the single ModuleDef registry (modules.ts): a module's optional
+// `titleKey` is the longer, descriptive tab title (e.g. "Channel manager"), falling back to its
+// short nav `labelKey` when the two are the same. Registering a new Page is then one touch (add it
+// to MODULES) instead of also editing a switch here.
 export function pageTitleKey(page: Page): string {
-  switch (page) {
-    case "feed":
-      return "header.account.feed";
-    case "channels":
-      return "header.channelManager";
-    case "playlists":
-      return "header.account.playlists";
-    case "notifications":
-      return "inbox.navLabel";
-    case "messages":
-      return "messages.navLabel";
-    case "downloads":
-      return "downloads.navLabel";
-    case "stats":
-      return "header.usageStats";
-    case "plex":
-      return "plex.navLabel";
-    case "scheduler":
-      return "header.scheduler";
-    case "config":
-      return "header.configuration";
-    case "users":
-      return "header.users";
-    case "audit":
-      return "audit.title";
-    case "settings":
-      return "settings.title";
-    default:
-      return "header.account.feed";
-  }
+  const def = moduleDef(page);
+  return def.titleKey ?? def.labelKey;
 }