feat(m5d): demand-driven deep backfill + per-user ETA

Per-user opt-in to full-history (deep) backfill so a new user's unique
channels no longer trigger a big shared-quota burst.

- migration 0007: Subscription.deep_requested (default false); seed admins'
  existing subscriptions to preserve today's global-backfill behaviour
- run_deep_backfill is now demand-driven: only channels at least one user has
  requested are deep-backfilled; recent backfill stays unconditional (cheap)
- estimate_deep_backfill ETA helper (quota-bound) surfaced in /api/sync/my-status
- POST /api/sync/deep-all to opt all my channels in; PATCH channels accepts
  deep_requested
- UI: per-channel Full history toggle, Backfill everything action, deep
  progress + ETA in Channels header and Settings - Sync

frontend/src/components/SettingsPanel.tsx

@@ -1,8 +1,9 @@
 import { useCallback, useEffect, useState, useSyncExternalStore } from "react";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
-import { Bell, Monitor, Pause, Play, RefreshCw, User, X } from "lucide-react";
+import { Bell, History, Monitor, Pause, Play, RefreshCw, User, X } from "lucide-react";
 import { SCHEMES, type Scheme, type ThemePrefs } from "../lib/theme";
 import { api, type Me } from "../lib/api";
+import { formatEta } from "../lib/format";
 import {
   configureNotifications,
   getNotifSettings,
@@ -335,6 +336,18 @@ function Sync({ me }: { me: Me }) {
     mutationFn: (paused: boolean) => (paused ? api.resumeSync() : api.pauseSync()),
     onSuccess: () => qc.invalidateQueries({ queryKey: ["my-status"] }),
   });
+  const deepAll = useMutation({
+    mutationFn: () => api.deepAll(true),
+    onSuccess: (r: { updated?: number }) => {
+      qc.invalidateQueries({ queryKey: ["my-status"] });
+      qc.invalidateQueries({ queryKey: ["channels"] });
+      notify({
+        level: "success",
+        message: `Full history requested for ${r.updated ?? 0} channels`,
+      });
+    },
+    onError: () => notify({ level: "error", message: "Couldn't request full history" }),
+  });
 
   return (
     <>
@@ -352,10 +365,18 @@ function Sync({ me }: { me: Me }) {
             </Row>
             <Row
               label="Full history"
-              hint="Channels whose entire back-catalog has been fetched. The rest backfill gradually (opt in per channel)."
+              hint="Channels whose entire back-catalog has been fetched, out of the ones you've requested full history for (opt in per channel on the Channels page)."
             >
-              {`${s.channels_deep_done}/${s.channels_total}`}
+              {`${s.channels_deep_done}/${s.channels_deep_requested}`}
             </Row>
+            {s.deep_pending_count > 0 && (
+              <Row
+                label="Full history ETA"
+                hint={`Rough estimate to finish full-history backfill of ${s.deep_pending_count} pending channel(s), based on the shared daily quota.`}
+              >
+                {formatEta(s.deep_eta_seconds)}
+              </Row>
+            )}
             <Row label="My videos" hint="Total videos available to you across your subscribed channels.">
               {s.my_videos.toLocaleString()}
             </Row>
@@ -382,6 +403,16 @@ function Sync({ me }: { me: Me }) {
             Sync subscriptions
           </button>
         </Tooltip>
+        <Tooltip hint="Request full back-catalog backfill for every channel you're subscribed to. Older videos + complete search fill in as the shared daily quota allows.">
+          <button
+            onClick={() => deepAll.mutate()}
+            disabled={deepAll.isPending}
+            className="glass-card glass-hover flex items-center gap-2 px-3 py-2 rounded-xl text-sm disabled:opacity-50 transition mt-2"
+          >
+            <History className={`w-4 h-4 ${deepAll.isPending ? "animate-pulse" : ""}`} />
+            Backfill everything
+          </button>
+        </Tooltip>
       </Section>
 
       {me.role === "admin" && s && (