Add older content exploration: channel page + home feed Rediscover mode

Channel page:
- "Explore older videos" button fetches 100 videos at a time further back
  in the channel history using yt-dlp --playlist-start/--playlist-end
- "Fetch entire history" still available for full crawl
- Backend: /channels/{id}/explore?page=N endpoint + playlist offset support
  in fetch_channel_metadata(start_video=N)

Home feed:
- New "Rediscover" mode: older unwatched videos (90+ days old) from
  followed channels, randomly sampled then re-ranked by tag affinity

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/api/index.js

@@ -48,6 +48,7 @@ export const followChannel = (id) => api.post(`/channels/${id}/follow`);
 export const unfollowChannel = (id) => api.delete(`/channels/${id}/follow`);
 export const indexChannel = (id) => api.post(`/channels/${id}/index`);
 export const indexChannelFull = (id) => api.post(`/channels/${id}/index-full`);
+export const exploreChannelOlder = (id, page) => api.post(`/channels/${id}/explore`, null, { params: { page } });
 export const followChannelByUrl = (data) => api.post("/channels/follow-by-url", data);
 export const setChannelAutoDownload = (id, value) => api.patch(`/channels/${id}/auto-download`, { auto_download: value });
 export const markChannelsSeen = () => api.post("/channels/mark-seen");

frontend/src/pages/Channel.jsx

@@ -3,7 +3,7 @@ import { useParams } from "react-router-dom";
 import { useQuery, useMutation, useQueryClient, useInfiniteQuery } from "@tanstack/react-query";
 import {
   getChannel, getChannelVideos, searchChannelYoutube,
-  followChannel, unfollowChannel, indexChannel, indexChannelFull, downloadChannel,
+  followChannel, unfollowChannel, indexChannel, indexChannelFull, exploreChannelOlder, downloadChannel,
 } from "../api";
 import VideoCard from "../components/VideoCard";
 
@@ -30,6 +30,7 @@ export default function ChannelPage() {
   const [search, setSearch] = useState("");
   const [activeQ, setActiveQ] = useState("");
   const [indexing, setIndexing] = useState(false);
+  const [explorePage, setExplorePage] = useState(2);
   const searchInputRef = useRef(null);
 
   const { data: channel, isLoading: loadingChannel } = useQuery({
@@ -94,6 +95,14 @@ export default function ChannelPage() {
     onSuccess: () => scheduleRefetch(45000),
   });
 
+  const exploreMut = useMutation({
+    mutationFn: () => exploreChannelOlder(id, explorePage),
+    onSuccess: () => {
+      setExplorePage(p => p + 1);
+      scheduleRefetch(20000);
+    },
+  });
+
   const deepSearchMut = useMutation({
     mutationFn: () => searchChannelYoutube(id, activeQ || search),
     onSuccess: () => scheduleRefetch(20000),
@@ -130,7 +139,7 @@ export default function ChannelPage() {
   if (!channel) return <p className="text-zinc-500">Channel not found.</p>;
 
   const isFollowed = channel.status === "followed";
-  const isPending = indexMut.isPending || fullIndexMut.isPending || deepSearchMut.isPending || indexing;
+  const isPending = indexMut.isPending || fullIndexMut.isPending || deepSearchMut.isPending || exploreMut.isPending || indexing;
 
   return (
     <div className="flex flex-col gap-5">
@@ -282,12 +291,19 @@ export default function ChannelPage() {
               {isFetchingNextPage ? "Loading…" : "Load more"}
             </button>
           ) : !activeQ && (
-            <div className="mt-4 flex flex-col items-center gap-2 py-4 border-t border-zinc-800/50">
-              <p className="text-xs text-zinc-600">All {videos.length} indexed videos shown</p>
-              <button onClick={() => fullIndexMut.mutate()} disabled={isPending}
-                className="text-xs text-zinc-500 hover:text-zinc-300 transition-colors disabled:opacity-40">
-                Fetch full history from YouTube
-              </button>
+            <div className="mt-4 flex flex-col items-center gap-3 py-4 border-t border-zinc-800/50">
+              <p className="text-xs text-zinc-600">{videos.length} videos indexed</p>
+              <div className="flex items-center gap-4">
+                <button onClick={() => exploreMut.mutate()} disabled={isPending}
+                  className="text-xs text-zinc-400 hover:text-zinc-100 font-medium transition-colors disabled:opacity-40">
+                  Explore older videos
+                </button>
+                <span className="text-zinc-800 text-xs">·</span>
+                <button onClick={() => fullIndexMut.mutate()} disabled={isPending}
+                  className="text-xs text-zinc-500 hover:text-zinc-300 transition-colors disabled:opacity-40">
+                  Fetch entire history
+                </button>
+              </div>
             </div>
           )}
         </div>

frontend/src/pages/Home.jsx

@@ -9,6 +9,7 @@ const PAGE_SIZE = 25;
 const FEED_MODES = [
   { value: "ranked",        label: "For you",      hint: "Ranked by your taste" },
   { value: "chronological", label: "New",           hint: "Everything in date order" },
+  { value: "rediscover",    label: "Rediscover",    hint: "Older unwatched videos ranked by your taste" },
   { value: "random",        label: "Explore",       hint: "Random from discovery pool" },
   { value: "inbox",         label: "Inbox",         hint: "New from followed channels since last visit" },
 ];