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/api.ts

@@ -123,10 +123,12 @@ export interface FeedResponse {
   has_more: boolean;
   // Opaque keyset cursor for the next page, or null when this is the last page.
   next_cursor: string | null;
-  limit: number;
   // Only set by the live YouTube search: "scrape" (InnerTube, no API quota) or "api"
   // (search.list, 100 units/page) — lets the UI drop the quota warning in scrape mode.
   source?: "scrape" | "api";
+  // NOTE: /api/feed also returns an echoed `limit`, but /api/search/youtube does not and nothing
+  // in the app reads it, so it's deliberately omitted rather than typed required (a lie for search)
+  // or optional (dead surface). Re-add if a consumer ever needs it.
 }
 
 export type PlaylistKind = "user" | "watch_later";