feat(channels): status filter + header full-history link + stable priority

- header: per-user "N without full history" count (channels_deep_pending),
  clickable with a hint -> opens the channel manager filtered to those.
- channel manager: status filter chips (All / Needs full history / Fully synced
  / Hidden); the header link deep-links to "Needs full history".
- fix: priority up/down is now an optimistic in-place cache update (no refetch /
  re-sort), so the list no longer jumps to the top and loses your scroll position;
  the new order applies on the next page load.

frontend/src/components/SyncStatus.tsx

@@ -1,12 +1,19 @@
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
-import { Database, Loader2, Pause, Play } from "lucide-react";
+import { Database, History, Loader2, Pause, Play } from "lucide-react";
 import { api } from "../lib/api";
 import { formatViews } from "../lib/format";
+import Tooltip from "./Tooltip";
 
 // Per-user status (not the global catalog): shows the number of videos available to *this*
-// user and how many of *their* channels are still being fetched. The pause control is
-// admin-only (it pauses the shared background sync).
-export default function SyncStatus({ isAdmin }: { isAdmin: boolean }) {
+// user, how many of *their* channels are still being fetched, and how many lack full
+// history (clickable -> channel manager filtered to those). The pause control is admin-only.
+export default function SyncStatus({
+  isAdmin,
+  onGoToFullHistory,
+}: {
+  isAdmin: boolean;
+  onGoToFullHistory: () => void;
+}) {
   const qc = useQueryClient();
   const { data } = useQuery({
     queryKey: ["my-status"],
@@ -23,6 +30,7 @@ export default function SyncStatus({ isAdmin }: { isAdmin: boolean }) {
   if (!data) return null;
 
   const syncing = data.channels_recent_pending;
+  const notFull = data.channels_deep_pending; // your channels without full history yet
 
   return (
     <div className="hidden lg:flex items-center gap-2 text-xs text-muted">
@@ -39,6 +47,20 @@ export default function SyncStatus({ isAdmin }: { isAdmin: boolean }) {
       ) : (
         <span>all synced</span>
       )}
+      {notFull > 0 && (
+        <>
+          <span className="opacity-40">·</span>
+          <Tooltip hint="Some of your channels only have their recent uploads. Click to open the channel manager (filtered to these) and request their full back-catalog.">
+            <button
+              onClick={onGoToFullHistory}
+              className="flex items-center gap-1 hover:text-fg underline decoration-dotted decoration-muted/40 underline-offset-4 transition cursor-pointer"
+            >
+              <History className="w-3.5 h-3.5" />
+              {notFull} without full history
+            </button>
+          </Tooltip>
+        </>
+      )}
       {isAdmin && (
         <button
           onClick={() => toggle.mutate()}