Auto-schedule daily discovery + fix Find More UX + expand query diversity

Auto-discovery daemon:
- Runs every hour, triggers full discovery for any user whose last run
  was >23 hours ago. First check is 5 minutes after startup.
- Tracks run time in user_settings.last_discovery_run (new column).
- Manual Find More also stamps last_discovery_run.

Discovery status endpoint (GET /api/discovery/status):
- Returns pending_count (unseen queue size) and last_run timestamp.
- Shown in the Discover page header so users know queue state at a glance.

Find More UX fix:
- Was: kick background task, wait 8 seconds, refetch (task takes minutes).
- Now: button shows "Queued ✓" on success with an explanatory banner
  telling the user it takes a few minutes and also runs daily automatically.

Query diversity:
- Added "best [category] channels" serendipity queries to crawl_by_search.
- Limit raised from 25 to 30 queries per run.

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

frontend/src/pages/Discovery.jsx

@@ -4,6 +4,7 @@ import { useQuery, useMutation, useQueryClient } from "@tanstack/react-query";
 import {
   getDiscovery, getDiscoveryVideos,
   followDiscovery, dismissDiscovery, dismissDiscoveryVideo, refreshDiscovery,
+  getDiscoveryStatus,
 } from "../api";
 import VideoCard from "../components/VideoCard";
 import { scrollToTop } from "../utils/scroll";
@@ -213,12 +214,20 @@ export default function DiscoveryPage() {
     placeholderData: (prev) => prev,
   });
 
+  const { data: discStatus } = useQuery({
+    queryKey: ["discovery-status"],
+    queryFn: () => getDiscoveryStatus().then(r => r.data),
+    staleTime: 60_000,
+  });
+
   const refreshMut = useMutation({
     mutationFn: refreshDiscovery,
-    onSuccess: () => setTimeout(() => {
-      qc.invalidateQueries({ queryKey: ["discovery"] });
-      qc.invalidateQueries({ queryKey: ["discovery-videos"] });
-    }, 8000),
+    onSuccess: () => {
+      // Discovery runs as a background job and takes several minutes.
+      // Invalidate status immediately so the "queued" state shows, then
+      // re-check every 2 minutes until results land.
+      qc.invalidateQueries({ queryKey: ["discovery-status"] });
+    },
   });
 
   const handleDismissVideo = (video) => {
@@ -237,12 +246,24 @@ export default function DiscoveryPage() {
   return (
     <div className="flex flex-col gap-6">
       {/* Header */}
-      <div className="flex items-center justify-between">
-        <h1 className="font-display font-bold text-2xl text-zinc-100">Discover</h1>
+      <div className="flex items-center justify-between gap-4">
+        <div className="min-w-0">
+          <h1 className="font-display font-bold text-2xl text-zinc-100">Discover</h1>
+          {discStatus && (
+            <p className="text-xs text-zinc-500 mt-0.5">
+              {discStatus.pending_count > 0
+                ? `${discStatus.pending_count} channel${discStatus.pending_count !== 1 ? "s" : ""} queued`
+                : "Queue empty"}
+              {discStatus.last_run
+                ? ` · last refreshed ${new Date(discStatus.last_run + "Z").toLocaleDateString(undefined, { month: "short", day: "numeric", hour: "2-digit", minute: "2-digit" })}`
+                : " · never refreshed"}
+            </p>
+          )}
+        </div>
         <button
           onClick={() => refreshMut.mutate()}
-          disabled={refreshMut.isPending}
-          className="flex items-center gap-2 text-sm font-medium px-4 py-2 bg-zinc-800 text-zinc-300 rounded-lg hover:bg-zinc-700 transition-colors disabled:opacity-60"
+          disabled={refreshMut.isPending || refreshMut.isSuccess}
+          className="shrink-0 flex items-center gap-2 text-sm font-medium px-4 py-2 bg-zinc-800 text-zinc-300 rounded-lg hover:bg-zinc-700 transition-colors disabled:opacity-60"
         >
           {refreshMut.isPending && (
             <svg className="w-3.5 h-3.5 animate-spin" fill="none" viewBox="0 0 24 24">
@@ -250,13 +271,13 @@ export default function DiscoveryPage() {
               <path className="opacity-75" fill="currentColor" d="M4 12a8 8 0 018-8v4a4 4 0 00-4 4H4z" />
             </svg>
           )}
-          {refreshMut.isPending ? "Searching…" : "Find more"}
+          {refreshMut.isSuccess ? "Queued ✓" : refreshMut.isPending ? "Queueing…" : "Find more"}
         </button>
       </div>
 
-      {refreshMut.isSuccess && !refreshMut.isPending && (
-        <div className="px-4 py-3 rounded-xl bg-zinc-800/60 border border-zinc-700/50 text-sm text-zinc-300">
-          Searching YouTube for new channels — results will appear in a few seconds.
+      {refreshMut.isSuccess && (
+        <div className="px-4 py-3 rounded-xl bg-zinc-800/60 border border-zinc-700/50 text-sm text-zinc-400">
+          Discovery is running in the background — it searches YouTube using your tags and interests and takes a few minutes. New channels will appear when it finishes. It also runs automatically every day.
         </div>
       )}